obviously excluding demons/demon likes(
etc), godlike beings(
etc) , void monsters(
etc) and 
(cus she litteraly channels her gods power).
taking in all their feats, powers, experiences and tools at their disposal(excluding using others) who would arguably be the most powerful in runeterra?
Has no powers that we know of, uses lamp post as a weapon, wins against much higher than his weight class because he is better at dueling and predicting his opponent. It's like Batman.
The problem is how you define "powerful" here. But there are some coming to my mind:
Genius and he built a time machine. Would probably be able to create way more deadly weapons if he wanted to and his rewind is something incredibly powerful.
In a world like Runeterra, an experienced sniper is actually extremely powerful. Basic soldiers like Garen, Darius and the like wouldn't even be able to move a foot before getting a bullet driven in their heads. To give context about her sniping skills: she managed to shoot a hare in the eye from a distance of 300 yards, with just a Bilgewater musket. Now she has a hextech enhanced sniper rifle, imagine the range she would be able to kill you from with that.
Ionian compaign, nuff said.
Jhin is used as a weapon by the cabal to terrorize both the Ionian government(s), and foreign governments as well. Not to mention that he was, and quite possibly still is, a serial killer(most likely still kills people for fun, as well as carrying out assassinations for the cabal). Not exactly powerful in the traditional sense, I'd say his influence on Ionia and foreign governments is something to be noted.
she's canonically 5"1' carrying a rocket launcher and a mini-gun.. able to ride missles like surfboards and single handedly laid siege to piltover..
a 5"1' girl did feats the equivalent to an army, by herself..
Syndra has the most raw power among "normal" ones IMO. Doesn't mean she can't be defeated but if I had to choose a character to destroy an army among these normals, I would go for Syndra. It is not like she didn't do it in lore. She freaking lifts ship with telekinesis.
Graves.
All he has is a shotgun. And the only fancy thing about it is the smoke grenade and its second fire (ult).
The dude relies on only that shotgun to fight. In a world of magic and pretty crazy tech, I’d say that’s the most normal.
It's honestly Kassadin. He's just a depressed guy that went to fight the void, the very thing that drove the darkin insane/scares Mt. Targon, because he lost his daughter. Most people die almost immediately to it, dude's balls deep and is still kicking.
Could you more clearly define what you mean by normal? Do they need to be human, or are Vastaya and Yorldes ok as well? I assume Shyvana is not allowed since she is a half dragon. Where are you setting the limit in terms of magic? Are you letting Ryze compete, who is essentially an archmage, Ezreal who has a Shuriman gauntlet, Twisted Fate who just has card tricks? A good example is Yorick, who is semi-immortal and has some magic capabilities in terms of controlling the black mist undead. Is he allowed? Or are you limiting it to no magic, pure human, like Caitlyn, Vi, Garen, Xin Zhao, Darius, Olaf, Gangplank, etc?
has enough power to destroy the world right
Isn't 
one of the strongest mortals (Or strongest?).
Depends what you're refering to. If mages are allowed, and . If they are not considered normal, 
.
Nobody mentioned 
Are you kidding me? lol
Jax. He can kill Aurelion Sol with just a cardboard tube.
No magic powers, no special relics, just martial experience and a lot of lip.
