I'm especially fond of the Restless Shadeling
, and managed to level it up to 25. I've found that nearly all the Undead types are pretty squishy, though. Thank goodness for their self-heals.
What's great about the shadeling is that it has two healing abilities: Plagued Blood and Death Coil. The latter is only 50% heals of damage dealt, but combined with the Plagued Blood, it can add up to a lot of healing (provided you don't miss and the opponent doesn't dodge or block).
On top of that, after applying Plagued Blood, any other pet you switch in will receive healing when attacking too (5 rounds).