Why Lee Sin is the most Overpowered Balanced champion
Lee Sin determines the fight. It's that simple. For all of the flame wars over the champion it's really just that simple. The design of the champion is one of the most frustrating and least interactive to play against. He could have a low of a win rate as urgot (through massive damage nerfs), and still **feel ** overpowered.
There is no control for those playing against him in the fight. Be it a gank or a lane, when you are fighting him, his ability to control if there is a fight going on at all, is what makes him so insanely strong. Think of it this way, what if you were in a gun fight (nerf guns obviously), but you could control when the guns were able to fire. Either both are working or both are disabled. It's similar to blitzcrank, though worse, it's control over when engagements and disengagements occur. Just make blitzcrank W a dash and make his E fling the target, and you've effectively replicated the problem.
In terms of input-ouput, lee sin feels awesome to play. Although when a Lee Sin starts fighting you, (even more so post level 6) all you can really do is hope that he is underestimating your dueling power, because if he isn't then the best you can do is tie.
Yeah, it's great to have a champion that's fun to play, but it's not good to have a champion that's fun to play, but not fun to play against. That means for 1 person to have fun, up to 5 aren't.