The best Myth pet is mostly decided by what talents it has. You'll definitely want Spell-Proof and Spell-Defy for defenses, Myth-Dealer for damage, and if you're still in the low levels you may want Fairy Friend or Unicorn as free heals are great at low levels. Unicorn even brings back your minion. Later you can try other pets like a quint-damage Mighty pet.
As for pet body, I highly recommend the Wandering Eye. The 8-pip AoE it offers is amazing before you get your Orthrus AoE. More importantly, it also comes with a Mythblade card, which remains important even after you get Epic Orthrus.