when pet battles were added, one of the first “rare” blue quality pets i caught was a Nether Faerie Dragon in Feralas. i named it Petal and it’s been my favorite ever since.
remains my go to for fighting flying types.
Wildhammer Gryphon Hatchling, (caught in Twilight highlands) are adorably bobble-headed.
i love my Arctic Fox Kit, (caught in storm peaks when it’s snowing.)
and Firewing (from the garrison’s daily, “mastering the menagerie” bags i think) is gorgeous.
Mechanical Squirrel is the only correct answer.
Not good at pet battling, usually busy on the trying to get better gear treadmill, but I have gotten quite a few over the years and played around with it…
The ones I like are the adorable, cute ones. Can’t remember the name of it, stonehide deer, or something like that, but it’s cute as a button and one of it’s attacks rams the opponent, sending it flying. Makes me laugh.
I want to squish it to death. 
I remember back when I frequented the old pet battle forums he’d always have chats with us 
On topic: Unborn Val’kyr. She helped me get my 5000 pvp pet battle wins!
I don’t really have a favorite as there is a lot of them I like to be honest. I did want to give a shout out to the Sun darter hatchling.
I like when they make quest chains for pets and make them fun to obtain.
Also it’s pretty much a recolour of the famous faerie dragon. I wish they would make a plush for the Sun Darter I would purchase it. Also potentially it would make other WoW fans happy they can obtain some type of plush dragon that looks like this again.
I see a lot of good pets in this thread tho.
Egbert fan here too. Was in a Guild Raid doing Naxx and was kicked for having him run around. 
Favorite pet that I generally have out “just because” is Poe my Gilnean Raven. Matches my mount too.
H
Poro, the best pet.
Why? I have a huge interest in birds. I study them alot, and almost everyday I go into my backyard to catch a picture of the occasional robin.
And Poro happens to be my favorite species of bird, the Kakapo! AKA. The ‘Owl Parrot’. Kakapos weigh about 2-9 pounds and are 23-25 inches tall. They are little land birds, that roam the forest floors of New Zealand. But they are sadly endangered due to human impact.
Fun fact: These little birbs live up to 90 years!
Mini Diablo is my favorite. I grew up playing Diablo and Diablo II.
when i did pet battles awhile back it was the Gazelle Fawn. nowadays only have a companion pet out to visually verify if my ISP is lagging me (pets will show rubberbanding when ping goes way up)
Wednesday, my Unborn Val’kyr. 
Without doubt, Smoochums is the cutest ever battle pet. He will always hold a special place in my heart because of his resemblance to my dear Persian boy who lived with me for 16 years. I like to think that in spirit my boy would be a cute as Smoochums.
Ragnaros. Because it’s the best cooking fire ever.
Did you invited him to a tea party?
Willy for the dirty emotes.
Unborn Valk
Ikky
The power team
Aesthetically the spirits from mop and the celestials
Lil, Rag. He will conflag critters… and he doubles as a cooking fire.
Sunflower!!!
Sprite Darter hatching. I always liked this combination of creature from my EverQuest days (fae Drake, long before WoW came out) and I always wanted the stuffed toy from the shop but was too broke to afford him way back when, and now he no longer exists.
My love for this pet is so strong, that I nearly teared up when I found out he is now a world drop and not a quest. You see, I moved back to the US after living 11 years in the EU and I no longer have this pet (nor any of my titles, mogs, achievements etc spanning to 2005 because you can’t cross region things and I play on a US account now to be with people in my timezone) . I have to start over from nothing and I can not force myself to farm hoping for him.
But every time I see someone with one, I make sure to /love him and I remember all my poor pets on my other account including the first baby murloc that are probably all alone and sad…
Panda Cub (I bought a laptop to get it!)
Mini Diablo (love the big flame “V”)
Mini Tyrael (love to /dance with him)
Argi (the adorable way he dances around)
Tito (I wish they’d redo him to an actual scruffy dog, but LOVE the tornado)
Baal, who i still need to get lol.