I really like that one quest in the game. I don't remember the name, but you'll probably recognize it from the description.
It's the one where there's a room with a door that you can't get through, but there are two lights next to the door. You have to go down one side passage, and at the end there's a way to turn on one of the lights. Then you come back to the middle, go down an opposite side passage and turn on the other light. Only then can you come back and go through the main door. I really like that.
Do you have a favorite quest like that?