Beta, you're like a burger....always have something to say.
And people like to eat you.
What about Renekton? That guy is just.. wow.
CC ruins him.
Renekton IS overpowered. The only reason Riot hasn't nerfed him yet is because they want to gauge his power correctly and not overnerf him to his release where he was useless.
I love Renekton but he is able to pull of insane stuff when he has solo top lane when played correctly and after that he wrecks carries in team fights making it a lose lose situation. He dives into the middle of your team, stuns your carry, and sticks on them. The carry has to focus him but he is so insanely tanky that without the carries damage the rest of the team will die to yours.
But Beta is right about Merc Treads. It will reduce the damage his ultimate does and reduce the duration of his stun, most teams will have much more crowd control than Renekton and magic resist is always worth getting.
He is overpowered early and mid game. Late game, he's a little off (You don't do enough damage, and you're just a tough dps, not a tank, meaning you can die easily if you're focused).

If I had to pick the single most powerful character in the game right now, it would be Renekton. His presence dominates the lane, it dominates teamfights, he does great damage while being incredibly tanky, he has hard CC, his abilities are completely spammable, he has a double dash...
The thing that makes Cho'Gath "seem" Over powered is that he can get resist items, such as FoN, or a Thornmail early, since his ulti gives him basicly free health. So his Mid-game is much more stronger as a tank, where as most tanks have to get a health item. Also, remember, CHo'gath's Ulti deals TRUE DAMAGE ( Ignores all magic and armor resists) so no matter how much resist you have, it is still going to deal the damage.
Now to the topic of "Tanks" in general they deal way too much damage than they should. In My opinion, Tanks are there to gain Aggro, provide items such as an Orical, defend the carry and CC the enemy team. But they end up dealing more damage from ability bases than DPS's.