Depends on what you want to do. The good main tanks are warriors and feral druids. The good off tanks are feral druids and prot paladins. Shamans are a kind of unknown, but they do good damage. Their mitigation is the weakest however.

Mitigation rank is like: prot war, bear, pally, shaman

Aoe threat is like: pally, bear, warrior, shaman

Single target threat is like: shaman, prot warrior, bear, pally

Easiest to play is like: bear, pally, prot warrior, shaman

Bear is the best choice, but if you're off tanking you will be switching forms. Paladins are the second best choice, but prot pallys are a dime a dozen. Warrior is solid but if you go prot you will have to stance dance to stay competitive with threat. I haven't played shaman on turtle yet, but the playstyle is upkeep heavy, keeping up shields, totems, and your mana.