I've always loved a lot of the raid boss fights in World of Warcraft, despite getting old after the 5th-6th time.
Nefarian in Blackwing Lair was good, every so often calling out a random 'Class Call' which manipulated all the members in the raid of the certain class into performing a less that desirable action.. Like all the Hunters breaking their bows, all the Mages turning everyone into random animals, the warriors switching to their damage stance and not being able to tank.
The Opera event in Karazhan is a good one, as well. It randomly picks from one of three classic stories (Wizard of Oz, Little Red Riding Hood, and Romeo and Juliet) and using those distinct stories to play off a boss fight. In the Oz fight, you need to fear the Cowardly Lion, use Frost/Water spells to rust the Tin Man, and use Fire spells on the Straw Man, and once all three of them are killed (and Dorothy/Toto), you have to fight the Wicked Witch who randomly spawns tornados that throw you into the air.