Coldain Prayer Shawl is my all-time favorite. If you do this around level 50, like I did with my chanter, it's quite a challenge in parts and a ton of fun. The tradeskill portion is the only thing stopping me from doing it on my cleric, but I may still do it anyhow simply because it really was a well designed quest.
Lucid shards used to suck, but I've had a couple of fun nights running my chanter around with my ranger friend getting those. The fact that we are level 70, I can charm mostly any mob we would need for tanking/DPS, and she can track definitely made it more fun. Plus, no one in the zones much anymore, so we don't have to wait very long to get two to drop.
The clicky snare necklace quest for Innoruk priests is also pretty fun.
More recently, the tradeskill trophy quests have been fun as well as DoN progression ones (DoN is nice because it combines some 'solo' tasks that you can solo or duo with group quest and raids but allows you to get some benefits without any raiding).
Favorite Old School One - Are you one with the wall?