Where is the cut off for what is considered a god in the league universe? In my opinion only 
, 
and the god of 
are the two confirmed gods in the universe that we know about. Everything else is just a powerful being that happens to exist and does things for reasons.
One of the main themes in the lore is challenging your preconception of what a god is. It's probably the most ubiquitous theme if you had to select a core theme of the League Universe. The fact that you have this question means you're reading between the lines.
The writers frequently present us with godlike figures and then peel back the layers of their character and show us their weaknesses and some of their pettier motivations that betray them as human-like entities and less like the traditional ideal gods portrayed in our world. The only reason most champions who can be called gods are referred to as such, is because so called "lesser beings" say that they are. It's just a matter of scaling and perception.
The Ascended are imbued with godlike powers, but their minds are full of mortal pitfalls. Azir suffers from pride, Xerath spite, Nasus hopelessness, Renekton insanity. The Darkin are much the same. Aatrox especially deep dives on it with his self-loathing and desperation. They only changed their bodies form but they are still very human where it counts. Not gods.
The Freljordian deities are referred to as Demi-gods and revered by certain tribes in the Freljord, but in reality they are all just powerful spirits. Ornns' tales are all dubious as hell they simply don't sound like things that actually happened, but rather more like people using gods to explain the occurrences of the natural world that they can't themselves. Ornn and Volibears' sibling rivalry, Ornns affection for his followers and his melancholy after their deaths. All more classic examples of Riot portraying gods behaving like humans to make you question their legitimacy. Not gods.
Lately we've been peeling back a bit on the Celestials too. These are beings who actually reside in the heavens. They manipulate events on Runeterra to suit their purposes. They can merge with mortals or control them entirely to enact their will. They embody abstract concepts that cease to exist when they do. They sound like classic interpretations of gods to us. Now with Pantheon we know that these abilities aren't hallmarks of their strength but rather glaring weaknesses. They don't control Aspects because they can they do it because they MUST. They need a go between since they can't physically exist on Runeterra themselves. Even the World Runes themselves are tools created by otherworldly beings so that they can craft the physical world. Because they couldn't create reality directly. And what happened when Pantheon assumed direct control of Atreus and went up against Aatrox? He died, the literal antithesis of godlyhood. Godhood as a concept is that we die... they don't. But while Pantheon did Atreus survived... and then Atreus KILLED AATROX HIMSELF AND ASSUMED PANTHEONS' MANTLE. Now he fights against all manner of "godlike" entities while championing the power of man. This is the most direct allegory challenging the concept of godly hood and omnipotence we have seen yet in the lore.
Aurelion Sol creates stars. Creation is a hallmark of godliness. Yet he's prideful and that pride led to his own downfall. He is now a "god" that lacks autonomy, which is laughably ironic and is by design. To make you second guess him. We also know that although he is old as hell, he isn't immortal he can die or be destroyed.
Kayle is the reverse trope a human who wants to be perceived as a god. She sees mortals as needing guidance and naturally self-destructive and she tries to annihilate her emotions and personality to differentiate herself from us. It's clear she struggles though, she felt guilt towards her fathers death, rage at Ronas' death, and she has conflicted emotions towards Morgana. You're not really supposed to think "oh is Kayle a god" she obviously isn't (she's not even a legit Aspect) but watching the same god complex quandary, but in reverse provides context for the other "gods". The properties of godliness she is imposing on herself are the same that are happening with all the other godlike entities. Here it's more pronounced and obviously forced to make us see how silly pushing for godhood is. Her righteousness, stoicism and lofty purpose cannot cover up her humanity.
The Watchers are other candidates for godlike entities. They come from a world of nothingness and are the biggest threat to Runeterra, the endgame. Their motivations are actually innocuous in a sense. Rather than for domination or because they are mortal enemies with the denizens of Runeterra, the Watchers want to destroy reality only because they don't understand it. It disorients them, it's antithetical to their very nature, and for this reason it must go. They even seem to harbor a childlike fear of reality. Their simple motivations lack the complexity of a true deities' "divine plan" and they react more like animals, blindly obeying instinct or like frightened children who cower from what they can't comprehend. Also just like the Celestials they cannot directly cross over to Runeterra. Last time it ended disastrously with Lissandra freezing them within the Howling Abyss, so they now use Voidborn to do their bidding. In-game Voidborn champs still have their unique personalities and sentience, but in the lore seem to be less than rudimentary beasts or drones. Not even sentient, made up of inorganic material and lacking in their own motivations. These crude mockeries of life are likely the best the Watchers could do to mimic life on Runeterra, because they lack contextual understanding of what life is. They aren't omniscient like a stereotypical god would be.
Kindred relies heavily on mystery to work as a godly entity. We know that all things must die and eventually they will all meet the Lamb and the Wolf. We don't know if Kindred exists BECAUSE of the cycle of life and death or if they created it. Cause or Effect? Kindred works on another level because for me at least I find it refreshing that their power scaling isn't based on might like the other gods. They can't face off against an army like Ascended or scorch the terrain of Runeterra like Aurelion with celestial fire. No, their power comes from them inserting themselves into the natural cycle. Associating with something you can't fight, can't run from, can't resist: mortality. What makes me skeptical for now at least for me is the aforementioned nature of Kindreds' relationship with death. Are they actual death incarnate or merely a messenger of something that would come regardless?
Which brings us to the final one. The entity that seems less god and more immutable law of the universe: Nagakaborous. We know next to nothing about Naga. Like Kindred the mystery shrouding the beings sometimes makes it seem larger than life. I do believe Riot will continue this examination with Naga in due time. In my personal interpretation is see a lot of parallels between Illaoi and Nagabourous and the Jedi and The Force. A religious sect assigning their own meaning to a property of the universe that doesn't recognize them as worshippers. Like a cult that worships time or gravity, the Jedi are guilty of putting their faith in a concept that either doesn't care about them or isn't even capable of doing so and this misplaced faith lead them to disaster. Wouldn't it be interesting if despite Illaois' beliefs and ability to channel Nagas' power, Nagakaborous wasn't even sentient? It just did what it did because it does. No divine purpose it just IS. That would be a more interesting direction to go in rather than to constantly deconstruct these gods, but then say Naga is the exception: the only real one.... There are no TRUE gods, because a god is a construct that exists in a mortal mind. Gods exist to give meaning to mortal existence because a god is everything a mortal isn't.
I now realize how long this post got with all of the examples. I definitely have to clean it up and make it it's own thread when I get back home. It's a topic that not to many people talk about, what makes a god a god beyond simply power scaling? Yet the game is so chock full of it that it may just be the core theme of League of Legends at this point.
Aurelion Sol isn't a god, nor is Kindred. The only actual god we know of is Nagakabourous.
In fact, while Aurelion Sol may be the single most powerful champion we've seen (Bard might be up there), Riot has this to say:
Aol Sol is, at best, a demi-god-- he serves only one reality and exists on only one realm. He mocks humans because they are as small to him as mites to us. BUT comparing him to Nagakabouros is like comparing a dung beetle to the theory of relativity. He's just pointing out he has a better view in the rug.
This is an interesting question to tackle, I think. Mrmeddyman wrote a great post on the godlike beings we've been introduced to so far that I really like but I'd like to go a bit further. It is curious to me that you specifically ask for a "cut off" and say:
Everything else is just a powerful being that happens to exist and does things for reasons.
A cut off would imply some sort of requirement for being called or considered a god whereas the quoted portion, in itself, seems to draw a distinction between "something that exists and does things" and a "god". What then is the primary difference between a god and everything else? Is it merely a matter of power? A matter of cosmological importance? A matter of why one exists rather than simply how one exists?
Waaarghbobo, in his posts about Nagakabouros and Illaoi, has hinted at similar questions. Look at the following posts here:
Aol Sol does not reflect a universal reality or truth. Aol Sol does not exist or travel between planes. Aol Sol can be trapped, contained, and controlled. Aol Sol can die in some ways. (Though does not age)
Ultimately he is "A True Dragon." An extremely powerful being. Is that a type of god? Meh... The definition of a god is pretty flexible (based on different cultural norms) So he could be depending on your point of view.
(...)
I did a lot of research on pre-christian faiths and Hinduism when creating this religion. Our real world pantheistic religions do not (and did not) believe in individual gods in a childlike way. It was/is a very philosophically complex view of the universe, where elements, and ideas are broken down and given constructs to hold greater concepts. Those constructs exist as dualities == both a truth and as mere symbol of a higher truth. (Thus the signifier is viewed with equal import as the signified)
Using those definitions (which is how Illaoi sees the world). A god is not simply "a being of a particular power level" it the reflection of an abstract truth.
And here:
Gods, spirits, and Aliens are a matter of perspective. Aurelion, Zoe, Ornn and others are sometimes worshipped as gods, that does not mean they are gods. Because that requires a definition of god.
Is nagakahbouros a true god or a manifestation of a universal truth..? Or is that what a true god is?
But granted he also said this more recently:
The more powerful a being is, the harder it is: to understand, to be worthy of its intention, or to communicate with. Probably naga is more powerful than the Volibear or Ornn (because a god > a demi-god) but which is easier to contact? which is more likely to come to your add?
There is a school of thought known as Ignosticism that basically argues that the term "god" has no meaning, that is, no clear set of properties attached to it and that, as such, discussions around it are useless until everyone can agree on a specific definition.
I think the intent here is to ask you, as an individual, what do you consider a god? And you'll see this in the setting too, those of the Targorian religion, for example, call the Aspects gods. Is that an adequate word to describe them?
Even Aurelion Sol in his color story notes how "god" is not a fitting descriptor of his nature. I suppose, in many ways, it's more about him attaching negative connotations of how mortals tend to worship him rather than the term itself being factually incorrect, but the point still stands. What I call a god and what you call a god do not necessarily align. Would a god even consider itself one? A lot of people just use an intuitive definition of what "god" or "holy" means.
For example, can a god die? Does a god need to be omnipotent? Does a god need to be alive? Does a god need to create the universe? Does a god need to be worshipped?
A lot of times in a lot of settings we are simply told "hey, this guy is the god of X" and never granted a lot of information about what exactly that means. Sometimes it's just the first guy from the first race which happens to be really good at this one thing.
With that aside, there's a few things to note:
the way I have interpreted the Kindred, due to comments calling them Spirit Gods (the same ilk of beings as the likes of Ornn, even if their worship is much more widespread), is that they don't just wear masks. They are masks. Masks of Death to be precise, a literal representation by ancient people who realized what Death was (more specifically, what it wasn't, a proper being with proper shape, only masks). They are only one cultural representation of a much larger truth (Death) and an exceedingly popular one at that (their worship spreads across Valoran as a whole and beyond).
is Nagakabouros sentient? Is it aware of mankind? If it's truly omnipresent, seemingly interwoven if not synonymous with reality, does it have a conventional will? Does gravity have a will, would you call it a god? Does that which is everywhere, in all things, have a greater impulse or a greater goal in mind? And note how the door is open for other Universal Truths: are they truly multiple, can someone so neatly divide the components of the universe, or are they so tightly woven together that to call them different things is in itself reductive and inaccurate (more of a tool for mankind to discuss it rather than an accurate objective description)?
by far. Even in his weakened state, he is by far the strongest.
Naga is the only god.
Bit of head canon, but here you go:
Kindred is both the strongest and weakest champ in lore, all things die, but some things are immortal.
Sol is not a good, just a powerful star dragon. Probably has the most brute strength of any champ.
Bard is at least as powerful and suggested to be more powerful than Sol.
Zoe can rival Sol with her dimension hopping and time manipulation.
Pantheon is ambiguously here.
Then we have the Demigod and Spirits, Voli, Anivia, Ornn, Janna.
Then we have the ascended.
Demons are ambiguously around here.
Then we have the other aspects, Leona, Taric, Diana.
Then we have immortal mages and Vastayan mages and yordle mages. Ryze, Lissandra, Neeko, Ahri, LeBlanc, Soraka, Lulu, Veigar etc.
Then we have normal human mages and normal yordles. Lux, Taliyah, Ezreal, Ekko, Kled, Tristana, Heimer etc.
And finally the normal human champs.
All categories in the box are pretty negligible in power difference really. And the voidborn are scattered throughout all the teirs.
I'd say 
is a god.
He exists outside the universe as we know it, observing most things but not intervening often, and then rushing into action when he feels like it.
As far as we know, there is no limit to what Bard can do, only on what he feels like doing.
Depends, especially since we don't have a clear-cut definition of a lot of the lesser powerful beings (ex. demons). Those three are definitely major deities, and so is Bard. But then there are also minor deities, which Riot lumps together as "spirits", which I think means that they're deities that are bound to Runeterra, rather than being free to move about the universe (or, in Kindred's and Nagakebouros's cases, already being omnipresent in the universe).
EDIT: See the posts below this talking about how the nature of a "god" is very subjective in LOL lore. They're describing it perfectly. The cutoff is basically where you, the reader, define it, or else there is none at all.
Who decides that the champion is God / not God?
as far as gods go there are none in game rn. We know the bearded lady/ Nagakabourous is worship by illaoi, that doesn't mean they're an actual god though.
Kindred is a story, a legend, like the grim reaper is in ours afaik. Death has so many variables in the lore it seems, i think the safest bet is to say kindred is like janna. they're an existance born from the believe in them. and thus she manifests in those areas.
A sol is just a big lizard tbh.
There are Demi gods in the freljord tho we have very little detail as to why they are demi gods especially considering asol could probably destroy em. If anyone knows about a god it's most likely bard i guess. but we also don't know what seperates bard from spirits like janna.
The real question is: If Aatrox is the “Godslayer”, can he actually kill Gods like Negakeburous?
also 
By a technical definition none of them are "gods" because by definition there can only be one supreme being without limitations. In other words, none of the gods mentioned in league lore are actual gods, because none of them are supreme, any of them can be killed (I think), and they all have limitations. A supreme being can be the only god. If there is something (other than self replication, self destruction, or self reduction) that it is capable of, then it is not a god because it is not supreme nor a source and end-all of power. Since it is defined by limitations, it therefore proceeded from something which made or created it. Therefore, neither Illaoi's god is an actual god, nor is aurelion sol a god. A god cannot being unwillingly or unwittingly constrained by a lessor being or it is not a god
