Well, from a pure power-gaming perspective, virtually all the choices are good. Why not Warlock? Dragon Shaman? There are tons of choices.

From a DM finger pointing perspective, there is no way this side of the Nine Hells I would let you do it. I mean, you could do it. He''d be intelligent. He'd also demand a full cut of xp and treasure as an equal party member; if you want a free Cohort, take Leadership or classes that duplicate it/give you minions. The Awaken spell specifically says you have no special sway over the Awakened Animal.