Picking up the legendary weapons in the Kael'Thas was pretty awesome!
Also nearly having a heart attack when I had my headset on full volume and hearing Anar'alah belore! bellowed at me.
Other than that, the class calls for Nefarion were great too and just the feel of the fight, it felt like the end of the raid. I know that sounds obvious, but fighting your way up to the pinnacle of Blackrock Mountain and having him fly in was cool.
More recently though I really enjoyed Blackhand too!