I've got 2.
Kael'thas Sunstrider. Was my first end-raid boss first kill with the guild I had joined a few weeks before, such an epic fight, still in that guild too.
Algalon. The awesome environment plus I loved his voice. Managed to get him down on 10, then a couple weeks later downed him on 25(Realm First). Still proudly wear my Celestial Defender title