What is fun to play (against)?
I feel like defining what champions are fun is a good way to being able to differentiate a champion that's simply overtuned vs completely OP and anti-fun. Now obviously "fun" is subjective, and my definition of what makes a champion fun to play against may differ from yours. But I have a feeling that we can at least agree on what champions are bullshit. As for myself, I put a champion into one of five categories when deciding how healthy a champion is for the game.
- Fun Tier.
These champions have enough of counterplay built within their kits, or are interactive enough to be considered fun to play against. These champions are the creme of the crop and what Riot should strive to balance their champions towards. These champions aren't necessarily underpowered, and in some cases they can carry games or be huge playmakers, and possibly are considered to be LCS tier. Being overpowered automatically disqualifies a champion from fun tier, but doesn't necessarily mean they're toxic, just overtuned, and can become "fun tier" once they're properly balanced. Lane bullies and assassins tend to be disqualified from this tier as well.
Examples: 
- Challenging Tier
These champions are still fun, but they'll defintiley make you sweat a bit. It's a tense kind of fun playing against these champs. Lane bullies, certain hypercarries, and slightly overtuned champs can fall into this range. These champs are more in your face than the fun ones, but they still have meaningful interaction and counterplay. Not top tier in terms of good champion design, but they're still perfectly fine gameplay health wise.
Examples: 
- Annoying Tier
This is the broadest tier of champions because it encompasses a variety of both fun and anti-fun factors, such as FOTM champions, overpowered champions that would be healthy when balanced, champions that hard counter a wide variety of champions, but still have meaningful counterplay in teamfights and against their counterpicks, etc. This is the hardest tier to define, but if I had to take a stab at it, I'd say it's a champion that you think about their worst part of their kits first and let out a deep sigh, but then think about them further and realize they aren't all bad. Most champions fall under here, and most players have different definitions for what they find "annoying."
Examples: 
- Toxic Tier
This is the cutoff point from when a champion goes from "annoying sometimes" to "unfun." These champions tend to have really binary kits that tend to go in the unfun direction from either being frustrating to play against, or being completely ignored. If you're skilled enough with a champion, you can overcome toxic champions, but most of the time you'll have to resort to counterpicks. Extremely overpowered champions and champions that you have to rely on counterpicking to beat in lane or in the jungle fall into this category. Lots of these champions are in Riot's sights for reworks, but aren't top priority, and are way further down the line than the absolute horrendous slough of walrus cankers that is the cancer tier.
Examples: 
- Cancer Tier
These champions are the worst of the worst when it comes to balance and counterplay. These champions are the ones that you cringe whenever the enemy team selects them, and lose a tremendous amount of self respect for your teammates when they play them. 99 times out of 100 these champions are not overpowered, simply because they aren't even allowed to be good. These champions are so unfun and non-interactive that even the mains of the counterpicks to these champions want to set themselves on fire rather than play against them. These champions are the bottom of the barrel and worst of the worst in terms of champion design. Counterplay against these champions is about picking against them, or hoping they screw up and getting really upset when they don't. These champions should never have been released with their current kits, and if their lore is bland or uninteresting enough, they probably should have been shelved. These champions are either slated for reworks, or dumpstered and left to rot until Riot can figure out how to fix their broken kits, after they're done going "what were we thinking when we released this broken unfun champion?".
Examples: old 
Well, those are my definitions of what is fun and not so fun, what are yours?
