I would have to say it's a tie so far between the encounter with Darl's party in LoLR and the first fight with Brazzamel in KoTR. I greatly buffed and optimized the red dragon.
I ran the encounter with Darl as suggested, at a moment when you're party might be at it's weakest. Not only had they spent much of their spells but the group seperated in the Den of Harrowdroth, and in wind walked Darl and his (in my opinion) coolest NPC party ever! Things turned ugly very quickly, without my using The Hand of Vecna to his utmost potential. Darl took all of the items for the required quest, plus pretty much all of the parties best gear, and left them alive. They had a whole new respect for Darl that lasts even now into 21st level when he still plays a major role.