The minotaur pet from Youkai in the tree of life is a good one. Mine is at teen and it already has maycast mythtrap. It is easy to get. It took me like 5 tries to get it.
I'd say the best Myth pet is the Lil' Medusa, which is the level 118 pet.
It gives a Mythblade, a stun all enemies for 1 round and a powerful medusa card. It has very good available talents that include a may cast myth aura, gives damage and resist. There are many more good talents available.
I have a Lil' Medusa pet that has 2 damage talents that gives 10% myth damage total. I would be available for hatching if you want to.
The best pet is the one you hatch and breed to meet your needs.
That said, you're going to need some strong pets to start out with.
There are some very good "free" Myth pets out there, including the Minotaur mentioned above (drops from Youkai); the Dreamsong Seraph (a rare drop in Avalon), and your school pets. But you won't earn any of these until at least level 35-75.
There's a Myth Treant for sale for gold in the pet shop in Wysteria that comes with a good card. It's a great place to start. Buy two of them and hatch them together until you get a good set of talents showing, then go from there.
If you have Crowns, the White Tiger and the Cunning Seal are both excellent pets available in the Crown Shop. The White Tiger is on sale through 4 July, so now would be a good time to get it if you're going the Crowns route.
Alia Misthaven, 120