For me, it was the Lich King, by far. The lore is amazing and he is who I wanted to kill since I really first started. Warcraft 3 built him up for me prior to WoW, and the fight, lore, and feeling was by far the best of any boss I've ever worked to kill. In my opinion, the Lich King was the final boss of WoW. While the lore has been interesting since then, it just hasn't felt nearly as epic and awesome as before, and it feels more like it's just dragging on now. I honestly felt nothing epic about Deathwing, and the fight itself was meh.