Depends how you level. If you quest and want to be able to deal with multiple mobs, spec a pet as Tenacity. Any pet can be any spec, but the specials are different and can bring survivability. Me, I leveled with a bear as Tenacity. Then grab a DPS pet for when you aren't concerned about the pet holding multiple mobs. Which pet doesn't matter as you level - the buffs are not really relevant to your leveling speed. If you PVP as you level grab a spider etc and spec it CUnning. This gives you 3 pets, pull out thr right one for the situation. At 90, get the pets that will provide buffs your raid doesn't have if y ou do 10 man raiding (25s will have everything covered).