If you're only interested in mythic pets, keep an eye on the horizon for the next few months because Abaddon (highest armor pet which also has max speed boost and 15% damage buff) will return before next cap raise. Generally, Slag is the best mythic pet because it has a solid AoE terrify (=~stun) in its arcane ability.

Personally though I don't think warriors need mythic pets, you're better off with legendary ones like little bear or ribbit/gyrm to buff your party.

Next season will start in a few weeks when the new arena goes live.