WW is THE Noob jungler. He's one of the few who can sustain in jungle with no runes or masteries, and his Ult = ezmode ganks. I don't like him for jungling in ranked however, because he really has no utility or gap closer outside of his ult. Once that's burned up, he's a lame duck. Also, if the enemy team is smart and builds QSS, he's useless.
Fiddle might sound good on paper, but in practice you get your ult interrupted more often than not, and he is super squishy. Until you get Zonya's he's extremely weak, and he can't gank until 6. He also can get counter jungled - fear or no fear. If you don't have blue buff you're basically useless. Very good in team fights tho.
Yi has extremely good clear times, jungle sustain and incredible damage and scaling, but not the best ganks when his ult isn't up because he has no CC.
Udyr if left unchecked can be virtually unkillable in late game, but his ganks are weak until he's pretty fed. Has the fastest jungle clear speed, no question. This usually means that lanes get no ganks while he just farms. I don't like this strategy, but a lot of people watch a certain person's stream and get delusions of grandeur.
Olaf is easy, but has bad ganks. Same with Cho, although you never see Cho jungle. I've seen people Karthus jungle, it's bad. Pantheon is interesting because he can gank from half a map away. He did see some use in worlds.
What you mostly see in the pros: Junglers who have a gap closer and an escape. Kha'zix, Lee Sin, Rengar. All have burst, sustain, clear speed, gap closer and escape mechanism. I would say they are all more complicated than the rest of the list.
My pick: Xin Zhao is very good easy jungler. Has short CD gap closer and knock up CC, and good sustain in the jungle. His ult requires some skill, that's about it. I think a lot of people at low elo prefer an ult execute, xin doesn't have that. I've effectively taken myself from Bronze 4 to Silver 3 by playing a ton of Xin Jungle, and I'm winning about 70% of my games at this elo as long as I can jungle. I'm 70 wins and 41 losses with Xin, and I just started playing lol about 2 months ago.
edit* Sometimes you really need some AP on your team, Amumu is extremely good in team fights and ganks very well starting at 3. Has no sustain and requires blue buff. I think Amumu is a better choice than Xin but I don't get as good of results yet. Diana is a good alternate, but Amumu ult = team destroyer.