Karazhan - it was just the perfect raid. Perfect layout, mysterious spooky tower, and it was Medivh's home. I'm a huge Medivh fan, so just exploring all the nooks and crannies of his old haunt was incredibly interesting. There are so many stories from Karazhan that were never told. Black Temple for much the same reasons - perfectly laid out, the bosses were challenging, and there were so many moments that made me either gape in awe (seeing Azeroth floating in the sky = brain blown) or wonder what exactly was going on. And ... oddly enough, the Sunwell. I didn't raid it when it was current content, my guild couldn't get past Brutallus. But I loved the way it was laid out, and I really liked the Sunwell trilogy manga, so it was neat to see it in game. And holy heck, Kil'jaeden emerging out of the Sunwell is still one of the neatest visual moments in the game.