I just wrote a huge in-depth post about each jungler and how each one is better than the others etc... Then firefox decided to jump to another link and erase everything I wrote here -.- So you'll have to excuse me if I cut it short now.... *Grrr*
Warwick is a good safe jungler, heals himself nicely, average jungle speed.. his disadvantage is he cant really gank lanes before Lv6 as he needs his ultimate.
Nunu is a great jungler, he has a build in Smite (and heal) with Q, a speed boost (and attack speed bost) for him and his lane ally with W... aswell as a very strong slow with E, making him a great single-lane ganker.
Olaf is probably the fastest jungler in the game, once he has blue he can spam his Q (by picking it up) and destroy creep camps, it also provides a good slow for ganking lanes, his disadvantages is he really needs to hit with Q when ganking (to slow) and needs his blue buff to jungle well.
Udyr, Trundle, Shaco, Irelia, Amumu & Fiddlesticks are also very good junglers, but I haven't played enough of them to go into GREAT depth, if your curious I could post the general theme for each of them (or more details), but for 'best', Its roughly;
Nunu, Olaf, Udyr, Trundle, Warwick.
Hope it helps atleast.