With multiple characters I have level'd to 18 to 20 all from questing, of course the random killing of mobs to complete those quests as well. My point is, doing quests is more efficient than going "xp'ing". Although, solo'ing with a priest is very hard around level 16+.
If you know what quests to do, and which not to do, you can easily gain ten levels (from level one) in, or under, six hours.
The only parties I had ever joined, up to level 18/20, were for Elite quests. Such as the Hordes Ragefire Chasm.
Edited, Fri Nov 26 09:54:48 2004 by Civilz