I recommend Nautilus on the following:
-Buttloads of cc, with an ultimate that is a near-infallible engage on a champion.
-He specializes in securing and preventing escape. Comes with a pull (q), slow (e), snare (auto-attack), a targeted knock-up that will follow its target anywhere (r), and a shield that doubles as an AoE on-hit DoT while it's up.
-Every ability he has scales off ap, but has highish base ratios on w, e, and r, qualifying him for bruiser builds if you need damage (RoA, Rylai's, Sunfire Cape, Abyssal Scepter, GA when I do it)
-Early/mid game monster (falls off late game, but remains a super enabler for the team)
-Ability to push with w
Flaws: Fatass can't run fast, though he can utterly decimate runners through cc and sustained magic damage. If you become skilled with him, you'll learn how to use his q to glitch through specific walls into other walls to pull yourself through, and you'll know when you can gain movement off walls rather than running, but his movement speed remains a bit on the low side. His clear time starts out sucky because his w breaks quickly, but after you level it once or twice, you cease to feel pain in the jungle and clear faster than average (recommend buying hunters machete at start, but selling it instead of making it into anything). Depending on his build, he can be a strong duelist against champions without high sustain or if they have a low hp pool. Weak against endgame ad carries and ad bruisers and sustain mages like Swain, strong against most other enemies.