Depends on the kind of playstyle you want to have but nothing beats a well geared, well played BM Hunter for fast easy grinding. It's probably the easiest thing in the game to do... Mark, send pet, hit it with serpent sting and then just auto/arcane rotation til dead. I am talking as someone who played as a BM Hunter from lvl 10 to 70 so my opinion will be a little biased, but still... BM is the best damage build, best grinding/ leveling build and the best raid DPS build end-game. Once you hit end-game you can choose between BM or SV for raiding builds, but MM only really has a place in PvP anymore along with SV and BM but most higher ranked (1800+) Hunters play as MM as far as I know.
Anyway I digress, BM for damage and leveling.... once you get to end-game pick your own spec, just make sure it's one of the cookie cutter builds as everything else just can't compete with them,