This is a fun thread!
I can never really decide on the best pets, and although technically I have a side account, I never ever log on to it, so I have to narrow down my "favorites" to only 4. That's pretty hard - I think I'd need 5.
Right now I think my dream lineup is...
Athelassie was a blumaroo when she was born and I just can't imagine her as any other kind of pet. So, since I can't afford Maraquan, I think the starry is my second favorite, and that's what she is.
The moment I saw the faerie ixi I knew I needed one. I created eowhinny AFTER I had the money for the PB, because I actually don't like regular ixis, they look mean! But the eyes are different on the faerie ones and.... *sigh* I love it, so she was made to be a faerie ixi. Now if only they made one that looked like sinsi, I might be tempted to change her.
Ethuiladdie was originally a blumaroo and became a lab pet, and so he's probably the only one I could imagine as anything else besides his current species. Right now he's just a green yurble, because TNT has not yet fallen in with my evil plan of instituting my Tyrannian Yurble image as the official one. Hmph! I'll have to hire a better hypnotist. Anyhow, if it ever becomes possible to morph a pet into a Lutari, I'll probably do that, and then be in agonies if they ever release a tyrannian yurble, trying to decide which one I want. I really love lutaris. Although it's probably pretty close between the island and green lutaris, I do like the green the best. (Plus, is island even released?)
Finally, Molly_G_Moo has been a baby kau since the day she was born (again, didn't make her until I could get the PB). Although her name is the one I like the least and could live without, she is my active pet, my "battle" pet (ha ha... with 44 HP...) and I think the baby kau is truly my favorite color.
A few others I really like...
Maraquan Draik,
Darigan Kougra, Mynci, Blumaroo