Fastest for me has been dungeon finder. In the first 40-50 levels, the RAF exp from completing a dungeon is more than an entire level. I often would ding twice upon completion. I did ragefire chasm at 15, and once it was over i was lvl 19, and I got 3 dings from the completion. If you can play a tank and a healer it will shorten the que times, but it's not that bad IMO. Often i would get a group that chain que'd and the only way it ended was that I leveled too fast, and the dungeon finder couldn't place us.