Komodo and crocodile have excellent regen. They are better at taking damage and will last longer when facing a tough foe that is whittling you down (damage reduction kicks in). The rattlesnake heals faster/dodges better.

I like the snake if you have a tough offensive form. Use your lion or similar to blast out some damage, then flee, shift to snake and move around a bit. Re-engage with lion at full hp again quickly. This tactic is viable if you prefer not to use wands as abs will retard your regen rates. Snake is pretty good defense form, as it can dodge as well as regen. I like it for exploring with groups. Its like a mini-mongoose that doesnt need to rest very often.

Crocodile allows you to explore and although tiring underwater..its pretty damn tough. Ive beat down water majors, with a crocodile. ROll is nice on water as well.