Here's a good question from the official forums: What's your favorite raid boss, and why? The original poster chose C'thun, from classic WoW's second-hardest raid, AQ40; Bornakk agrees. I haven't actually seen C'thun, but from what I hear it was both incredibly frustrating and incredibly awesome. Crygil picks Doomwalker, but he might be a little biased.
As for me, picking out of the fights I've done, I'm going to say Hodir. I don't remember classic raiding very clearly, and I only got through Karazhan and most of Zul'aman in BC. Ulduar is probably my favorite raid instance out of all the ones I've done, and the Hodir fight is really fun and demands a good amount of coordination from your group. What's your pick?