Nasus

I think that Nasus is a little bit too unstoppable. His lifesteal passive makes harassing him pointless. His ult makes ignites and HP regen debuffs lose a lot of value. His Q is infinitely stackable and affects turrets as well (such as being able to two-shot a tower - even the nexus turrets). He has no real escape, but can soak up damage and build tenacity to just roll through the entire team, drop a few Q's, and get out with full HP. He has no real counterplay as far as his kit goes (even if you kill him multiple times, he just TP's back in, farms his Q and eats all harass and just builds his HP back with his passive. His wither is such a low cooldown and he has an active armour shred - though that isn't the OP part. What makes him OP is his ability to do what Cait's headshots used to do (proc on towers) times TEN. There is really no stopping him if he has an ADC that is worth anything. You try and take him down, spend an hour doing it, and get out-DPS'd by the enemy adc