This is a topic I know will get me into trouble. Classic WoW is known as "classic" for a reason, after all. Being the opening foray into the MMO that has since become the standard for the entire industry, WoW's first incarnation is filled with memorable encounters. The raid bosses we battled back then are the yardsticks by which we have judged pretty much everything that has followed.
I know it's tempting to say something like Ragnaros or Onyxia or C'Thun, because those are the big bad end boss fights that remain etched in WoW players' collective consciousness. I've always tried to stay away from the obvious answers, though, because, well, they're obvious. I don't think there's anything particularly interesting I could write about Ragnaros or Onyxia that hasn't already be said better by someone else. When I sat down to brainstorm about this topic, I ran through many of the classic bosses, testing my memories of them, and after a while I hit upon what I knew would be my personal answer to this question: Vaelastrasz the Corrupt.