Hunter all the way. With boas you can one autoshot any mob in range as you quest along until the level 40 ish range. In cata I leveled a hunter 1-42 the first day, 42-68 the next day, 68-81 the third day, and finished getting to 85 on the 4th.
If you mean fastest /played time as opposed to real life time, I would suggest doing the daily quest on your monk for a few minutes every day even if you don't intend on leveling it yet. The 50% xp buff stacks so you can have many many hours of +50% xp ready for when you wanna have a leveling session with the monk.