Well, I have just a plain old dragon, the type you can buy from just about any pet shop, but I really like it because it was my first pet (besides the piggle) But since a dragon isn't a very valuable pet, my absolute favorite is probably my starfish, Chico, or my Sea Dragon, King Xena. Again mostly because I've had them a long time... but at least these ones are a bit rarer! Dakota Ravenmask Level 51 Cheyenne Ravenmask Level 9
I've got about 60 pets and my favorite is my frankenbunny. I am a storm and so is Fluffy (my bunny.) He gives storm critical rating and a critical hitter. He is leveled to epic but never got energizing battery (may cast healing current) but he does have it as a card. I enjoy him lots!
Seeing that i am my favourite pet has to be the lvl 98 avenging fossil pet. That is a crazy pet for death because mine is a 7th generation mega that gives 16% damage, may cast infallible, may cast virulence, may cast death blade and finally 11% resist. It may have no heals but heals don't really matter for me seeing that i am lvl 75. I am done with pet hatching and training but it was worth it.
The Avenging Fossil is too good not to use, especially with the right talents. The main thing about it is the cards. It gives a Deathblade Item Card, which stacks with normal and sharpened blades. There's also the Feint Item Card, which also stacks with its normal and potent versions. Having a third Feint and Deathblade is immensely helpful.
My favorite pet is the red panda. Mine is called Fluffy and is a ancient right now but so far i have been working to get him to be a mega. My favorite part about him is his may cast spirit shield and may cast spritely.