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.
I missed out on raiding for classic WoW, but I did go back and do as much of the content as I could in early TBC. One of the reasons I remember Vaelastrasz so vividly was that he was the first boss we encountered, as Karazhan-geared raiders, that was still actually hard. Vael wiped us over, and over, and over. To be fair, we did not have a full 40-person raid team, but we hadn't anticipated needing one! We did end up bringing in extra people to get him down, and even then it was close. Between blowing up our healers and chain cleaves, Vaelastrasz still proved to be a content gateway and a truly exhilarating fight.
Vaelastrasz first appeared in WoW as a friendly NPC hidden in Lower Blackrock Spire. He gave the quest to assemble the Seal of Ascention, which granted access to Upper Blackrock Spire. He was also a sworn enemy of Nefarian, and vowed to hunt him down and defeat him. Well, poor Vaelastrasz certainly hunted Nefarian down, but it was he who was defeated.
When you enter the Shadow Wing Lair of Blackwing Lair, you can witness the moments immediately following Nefarian's victory over Vaelastrasz. Nefarian, in his human form, is using magic to corrupt and turn the red dragon. He orders Vael to kill the raid and departs. Vael, knowing that he cannot fight Nefarian's magic, uses the last of his free will cast Essence of the Red on the raid, a buff which lasts three minutes and is essential to completing the encounter.
Something worth remembering is that threat generation and retention was much more difficult for tanks in those days. The Vaelastrasz fight lasts about three minutes with a tank guaranteed to die about once every 45 seconds. Thus, at least four tanks were required, all lined up in a threat-producing queue. Maintaining this balance of enough but not too much threat could be incredibly difficult, and the transition between tanks was often fraught. For a DPS race fight, this meant that the balance between producing as much damage as possible while not pulling aggro, particularly as one tank was getting ready to die, was even more important than normal. As if that weren't enough, there was also the typical dragon chain-cleave to keep in mind, which could result in the instant death of all tanks if folks weren't careful about their positioning. The result? A heart-pounding gear check of a fight that tested tanks, DPS, and healers alike, and also tugged at the heartstrings as you were forced to destroy a character that had been your ally in the past. For those who completed the Scepter of the Shifting Sands, Vaelastrasz also became one of your quest objectives, for he was the dragon in charge of safeguarding the red flight's Scepter shard.
I expect my choice here to raise some eyebrows. When people talk nostalgically about the memorable raid bosses of WoW, Vaelastrasz isn't often on the list. But I can't deny that he holds a special place in my heart that is also bittersweet, because he was the boss that broke the first Horde guild I joined. They recovered for a time by merging with another raiding guild, but the entire thing fell apart completely in Burning Crusade, and I finally pulled my remaining toons out in Wrath of the Lich King. I don't hold it against you, though, Vael.