Udyr and Shyvana have the best clear times, but fall off late game.
Nunu and Warwick have some of the best sustain, but are relatively useless early game.
Nocturne and Warwick have the best ganking, but don't really shine until mid-to-late game.
Shaco is the best at counterjungling, hands down, but has the single worst late game potential out of every Jungle champion.
Lee Sin, Xin Zhao, Corki, and on and on are all viable picks.
Every single jungler in the game is the best in Season 3 if you know what you're doing. My suggestion is to just be careful in Champion select. In Blind pick you never know if you're going to be up against a Jungler that will rip you to shreds (Udyr and Shaco are terrifying Counterjunglers); in Draft Pick you need to pick a Champion that either counters their Jungle (if they get first pick) or can be easily supported by your team when they counter yours, and you need to know if your Team Composition supports the kind of Jungler you're picking. If the comp is a passive comp, choose a Jungler that can stay in allied Jungle and farm safely (Warwick is my preference due to his strong 1v1 skills and high sustain). If your comp is aggressive, pick a Counterjungler (I cannot stress how good Udyr and Shaco are at this). And always, ALWAYS do these two things:
Take Smite. It's invaluable for both counterjungling and deflecting the enemy jungler from doing the same. And:
Buy Wards. It'll prevent ganks, keep you in the know when the enemy Jungler is crossing the river into your territory, it'll let you know when the enemy is going for Dragon/Baron, etc. The support simply buys wards; you spam them across every inch of territory you can.
EDIT: If you're on Twisted Treeline, play as Warwick or Shaco. You will simply dominate the early and mid game, and if you reach late game, you'll be so fed that even Shaco can wipe the entire team.