Prove Your Worth is undoubtedly my favorite. I run this at least once every life. However, I realize that not everyone shares the same high opinion of this quest as I do. It requires skill and wits to maneuver the challenges. The icing on the cake is the "talk like a pirate" challenge at the end. The whole darn quest is just one fun little challenge after the other. I love quests like that. I love the acrobatics and mazes like those in The Pit, The Coalescence Chamber, and Spies in the House as well. I realize not everyone enjoys such quests as much as I do. Some just want to kill monsters and run to the end, but I find that boring. Prove Your Worth breaks up some of the low-level monotony.
I do like the massive puzzle in Ghost of a Chance, so it gets 2nd place for me. I also like that it is optional but grants bonus loot and great XP. So those who want to do it are highly rewarded and those who hate such puzzles can skip them.
As for my least favorite: Guard Duty, without a doubt. I don't even have to think about it. Not only is it incredibly easy, even soloing on elite, it's just boring waiting for monsters to spawn. If it were fast-paced and monsters spawned more readily or there were a greater degree of dungeon scaling, I might enjoy it more, but I usually skip it. BTW it is also TERRIBLE XP/Min.