I'm gonna bring in League of Legends and mention Tryndamere.
Now, before anyone gets their tighty whities bunched up lemme explain my reasoning.
I don't take issue with the fact he does a lot of damage, he IS a damage dealing character.
I don't take issue with the fact he can go completely invincible for 5 seconds without any cost (Ok maybe a little but that's just because it seems cheap, not OP)
My problem lies in the combination of said powers
For each 1% health he's missing he gains attack power.
For each 1% Fury he has he gains Crit rate.
When he uses his ultimate, he has 100% fury, and <2% hp, and CAN'T be killed for 5 seconds, within these 5 seconds he can (if used well) win a 5v1 teamfight.
That alone STILL isn't my issue, my issue is the fact that he can use this WHILE STUNNED (and it has no cost) so there's NOTHING! you can use to stop him from using it, at all.
And he can do this every minute.
In a game where matches can last well over an hour, having this ability finish recharging every minute, it all adds up to OP.
Now, the usual argument is "Just stun him till he finishes using it" which doesn't hold ground, a full 5 seconds of stun is a lot of stun, being able to soak up ALL of that while his teammates wail on you means he's already fulfilled the role of 'tank' better then most any character in the game, a character with one of the highest DPS in the game should NOT be able to tank better then most other characters as well...
Another argument is "Kill Tryndamere last" but that still doesn't hold up, a Trynd built for damage WILL be able to kill any one of your team members in a heartbeat, thats what he was designed to do, so by not attacking him you lose anyway.
I suppose Tryndamere himself probably isn't OP, none of the characters are, but he's too easy to MAKE OP...
I KNOW someone's gonna disagree, probably with "Hurr but Trynd isn't used much in tournament play" or "Hurr, only newbies can't take Trynd" but at the end of the day, he needs a little rebalancing, even a small change like "His ult doesn't give him max fury" (which he can use to heal with by the way) or "His ult can't be used while stunned" would peg this down from OP to just plain old P.
/rant I suppose.