To me the best old raid is split between Karazhan and the Black Temple. Both raids offer a ton of interesting fights, and the different zones of the raid really flow amazingly well together. In Kara you feel like you're traversing through a mad wizard's tower (something iconic in the fantasy world); and in the Black Temple you feel like you're making your way up to meet the lord of the world, and all this minions are destined to stop you.
In many ways Icecrown Citadel offers the same feeling as the Black Temple does -- it's clearly the final raid and the most important thing you need to do in the expansion. However I don't feel that all the bosses in ICC flow and fit well together. Their purpose, at times, feels a bit disjointed. It's still a wonderful set of encounters (and individually, I feel some of WoW's strongest encounter design), but to me... personally? It just isn't my favorite.
What's the best old raid for you guys? Where do you go and just get all excited again over how awesome the place it?
You are not prepared!