The best pet by far is the pixie. The loomi money collector works on easy, but really does nothing you can't do yourself. The lizard sounds nice, but it is never necessary to bring down flying enemies since the best tower, the boomerang, works on both ground and flying enemies. The pixie will give you a continuous flow of mana throughout the level and will be indispensable. From my experience in hard mode, beating the first boss without 7 freeze traps (only attainable with the pixie) would be impossible.
If you are playing on easy, it really doesn't matter what you pick.
the loomi is nice cuz you get the money the moment it falls. if you get the right skills upgraded with loomi its a piece of cake.trouble is the right stats.and im not telling! :P
I picked the loomi. I'm also too lazy to pick up money. Also it saves much time and you don't have to worry about picking up money and you can focus more on building towers.