Currently a tank.

1-80 dungeon, 80-100 quest/dungeon.
100-110 legion assaults/dungeons(even faster if you find a 101 twink)

Not at all, unless you're rafing with someone.

Do you even play this game?
Or are you one of those subbed doing pet battles with your gf and commenting on how to beat mythic guldan on the forums?

@OP Yes, dungeons are the fastest way.
There's some tricks to level faster at level 90 if you bother you can YouTube these, and at level 100 if it's your first time, just play the game, if not you can dungeon/invasions/quests by that order.

yes dungeons are the fastest but dont just sit in town waiting for one especially if your a dps quest while you wait

no quests are faster for sure

if you have quests in the dungeon it is for sure though

lol. all dungeons in the game have been revamped to include 2-5 quests right when you walk in