Cho' Gath would be the best base level 18. He does plenty of CC compared to the rest, with +Health stacks from his ult, which can make his base health around 3000, at lv 18.
To sum it up.
Pros:
+Insane Crowd Control (Silence + Knockup)
+True Damage Ultimate
+Health stacks
+Highest base damage in the game.
+Sustain from passive / Ultimate
Cons:
-No mobility
-Low armor + magic resist
-Very predictable
Compared to the other tanks, such as Garen, Amumu, and Shen, they can only deal small amount of damage, with only one huge CC. However, because of the lack of items / runes / masteries, they are extremely squishy, and unreliable.
So basically, Cho' Gath offers the most out of all the tanks.