I don't know why but Garalon sticks out as my favorite early tier boss fight. Perhaps because it was the mechanics really allowed you to see the difference between a good tank and a bad tank but I just really like that fight.
Primordious is my favorite Throne of Thunder fight. I thought the idea of being transformed and killing oozes was fantastic and the saurok casting animations I freaking love.
SoO? I don't have a favorite fight. Really out of any of them if I have to pick? I'm going to say Nazgrim. I say that because the adds are frequently more lethal than the boss.