Kael'thas in Tempest Keep at level 70 in appropriate gear was by far my favorite fight, it had complexities as well as unique mechanics. The weapon mechanics made the fight, and the intensity of the fight with the weapon and second adviser phases made it a bit of a test. Then the pyroblast mechanic was the first introduction of a shield that prevents interruption if I'm not mistaken and led to many more fights with similar mechanics (including trash in Blackwing descent).
My favorite raid is The Molten Core, despite going there for 16 weeks straight now (Farming the Eye), I still enjoy seeing Ragnaros being summoned. The fight it's self is something else, I'm a Orc Furry Warrior lv85, I can solo it easy as cake, but the second I hear "Taste the Flames of Sulfuron!" I panic. The fight is easy to win but if you let your guard down it's over.Right now my favorite dungeon boss is Ozruk in The Stonecore, it's a cool fight and I like hearing "Break yourselves upon my body. Feel the strength of the earrrrrth!", that and I like mining bosses.
Heigan the Unclean! The dance was just so fun :)
I have two favorite encounters, and they are for the same reason... the winner has to be The Lich King fight on the Frozen Throne. The Gunship Battle is fun, because it adds to the 'Epic feel' of storming the Citadel with the help of Muradin Bronzebeard, battling to the top of the spire, defending against the cowardly Horde while they try to blind side us. But it still pales in comparison to marching to the very top, calling upon the aid of Tirion Fordring and defeating the Lich King, battling Val'kyr who try to drag your allies out of battle, laying waste to droves of Drudge Ghouls and Shambling Horrors, defeating Raging Spirits ripped from your own forces, and then to have it all stripped away as The Lich King kills everyone in order to raise us as his own personal army of the strongest heroes he's faced only to have it all snatched away from him by Tirion. Because of the epic feel, and the way you are immersed in the fight, defeating The Lich King is the best encounter that I've experienced to date.
Picking today's winner real quickly then a new drawing (the last one) is going up!!!