learn to play every jungler
memorize their routes/skill orders/paths/times.
learn to play against every jungler
learn to fit your jungle champ into your team comp, and against enemy team comp.
there is no one jungler; learn them all.
to answer your question:
all very powerful junglers.
if you want some advice from a player who basically *only* jungles: nasus jungle is fun as F*CK
EDIT: i forgot some things
learn to counter jungle
learn what to do when counter jungled
and i guess most importantly; learn to make GOOD decisions.