Essentially:

Questing in Instances > Kill Quests > Instances > Other Quests > Collection Quests

Questing in Instances - You get XP for the mobs, good XP too & XP + rewards from the quests.
Kill Quests - Good when you can find them but I don't think it'd be possible to level all the way up with just these.
Instances - Good XP from mobs.
Other Quests - Providing you know where the pickup and drop off is and can do several things at once they aren't bad for XP especially once you hit 30+ (I had 2330xp for running across a camp for one delivery quest).
Collection Quests - Actually give good XP, but unless you have a very good setup for dealing with looting/per character they are a pain to do. Plus unless each mob drops the item each time you end up killing vast swathes of mobs... Only really good if the mobs are elite.

Hope that helps.