Death Knight - unholy seems by far the best.
Druid - personal one, I would level as feral.
Hunter - no clue, no experience with these guys
Mage - No clue again, but arcane seems pretty good? :/
Paladin - prot <3
Priest - disc or shadow?
Rogue - Combat or Sub...maybe Sub
Shaman - Id say enh, here. Even though I leveled my shaman ele.
Warlock - affliction imo
Warrior - Id say fury.
What do you guys think the main specs will be for leveling?