lee / udyr really.
both easy to jungle with, both have insane gank options, neither need gear to do their job well, which is a really key piece to the kit, if your lanes suck / fail and you have to spend a bunch of time out of jungle or all your time in gank mode, but they end up stealing the gank kills you won't have the gear you need. This leaves you behind at end of game.
Lee and udyr don't have this problem, gear is just bonus for them. While noc, nunu (pointless nowadays he better used in lane), trundle, GP, skarner, can easily fail in jungle and it not even be their fault.
Rammus is a solid choice as well, his move speed lets you get around the map fast enough to save a champion in trouble.
Noc's gank is weakest of all, it's easy to predict only is useful if the champion is already below 50% health, and he has to have lots of assistance from the lane, meaning he can't dive in when his lane partners have been harrassed to near death, and do anything about it. Other junglers though can go in and make their own way on ganks.
Skarner also ends up in this situation, he can gank top easy, but bottom lane if they are smart have no problem countering his gank option which relies on getting his ult off.