Stonewall's tier list is probably a good resource to go by:
http://na.leagueoflegends.com/board/showthread.php?t=325897
Tier 1 – Udyr, Lee Sin, Trundle, Nocturne, Fiddlesticks, Cho Gath, Skarner
Tier 2 – Warwick, Riven, Shaco, Jarvan, Nunu, Xin Zhao, Amumu, Gangplank, Maokai,
Tier 3 – Master Yi, Olaf, Malphite, Tryndamere, Shen, Yorick, Rammus, Gragas
Tier 4 – Alistar, Akali, Graves, Sion, Galio, Irelia, Kayle, Renekton, Soraka, Nasus
Tier 5 – Mordekaiser, Karthus, Pantheon, Taric, Wukong, Jax, Evelynn, Orianna, Talon
Anything under Tier 3 should be avoided if possible.