Nunu is strong, however he is also Niche, because his kit forces him into more of a support jungler role. He fits on a team with an ally who can abuse Blood Boil- support-tank Nunu in the jungler with a Vayne on his team is really, really good. Tanky-dps who synergize with attack speed also like him, Shyvana and Shen come to mind.
I've seen Udyr do well on this map. And Lee Sin is, as always, strong but difficult to master.
Also- if price is a factor, Nunu is only 450 IP, and Udyr is 3150, while Lee Sin is 6300. So, get Nunu, maybe get Udyr, and wait for Free Week to try Lee Sin before buying if you don't have IP to burn.