Best pet for Magery Mastery, is a Serpentine Dragon. They can spawn with up to 1,040 INT (way over the 700 cap), which also means up to 1,040 Mana right off the bat. The higher INT not only improves their mana regeneration rate, but it also boosts the damage of their Mind Blast significantly. They're like the Arch Mage of pets. However, even with 1,040 INT/1,500 Mana/30 MR/120 Med/120 Focus, a MM Serpentine Dragon will eventually run out of Mana if it continues blasting, and even with 1,040 INT/120 Magery/120 Eval it's damage output still doesn't compete against that of a 120 Chiv+AI pet.
The only time my MM Serpentine Dragon gets use, is when i'm hunting down Orc Ships to level low-slot pets on a Dread Pirate. I'll use the Serpentine Dragon to teleport onto the Orc Ship and kill the Orcs, then i'll recall out, stable him, and grab whatever pet i'm gonna level on the Dread Pirate.