If you are using RAF, running dungeon finder is the fastest way. I ran 3 priests in ragefire chasm right when I hit 15, and between the dungeon quests (that are all right by the entrance now) and the bonus exp for dungeon finder, I was 23 when the dungeon was over. The whole run was about 15 minutes, to get 8 levels, which is one level every two minutes...

Without RAF I can't be sure, but with the changes to scarlet monastery, It doesn't seem to be as useful for boosting any more, seems to me that there are fewer mobs, and in the new armory, the section with archers with knockback sucks, as the knockback works against an 85.

I would just quest. Collection quests are easier now, with AoE looting and items drop for every party member.