I doubt simple naming a quest is a spoiler.
Anyway my favorite was the only cure and my least favorite was boethiah's quest. Wow we are exact opposites.
It (somewhat) isn't, but a topic of discussion like this can easily end up discussing the quest details. Anyway, it's already been moved.
My favorite one was the Wabbajack one, simply because I love Sheogorath. I've yet do do even half of the fifteen daedric quests though. I'd have to add the Spellbreaker one as well. ONLY because that dungeon at the end was the longest, most difficult/challenging dungeon I've had to deal with. I did it on Master difficulty at like level 15 on a sword and board character with NO crafting skills. Very fun indeed, but only due to the circumstances. Plus, people kept vomiting on me. What's not fun about that? (Okay, that's not really fun at all.)