I don't raid, but a co-worker of mine does. He talk enthusiastically about one of his favorite raid boss encounters in the expansion: Hydross the Unstable in Serpentshrine Cavern.
As he explained to me, the encounter is a particularly intricate dance of timing and coordination. From what I gathered, Hydross switches damage types, so the raid must have two main tanks with two different resist gear sets on. When Hydross switches, so must the tanks. Also, Hydross must be dragged about the cavern at timed intervals which also spawns adds.
I believe the high level of coordination among the raid was what made this boss a favorite of my co-workers. Now I put it to you, gentle readers: what is your favorite raid boss encounter and why?