I'd recommend a Hydra. I was extremely disappointed by how weak the level 42 Hydra spell was, but with my Hydra PET, the spell is stronger, therefore making it more useful. Another idea would be the Crabling pet.
I'm L60 on Balance and my own favorite pet is a Talented Piggle. After a few hatches I got one with spritely, balance damage boost, balance accuracy boost and health gift. Add in the fact that it comes with a dragonblade (+30%), it is an awesome pet even if it's not the best LOOKING one.
Now, I'm trying to get those talents (mostly spritely) into a Jade Oni