Tunat'Muram Cuu Vauax, Everquest - Gates of Discord.
For the first 10% of his health, he hit for 6,500 damage per hit (every, oh let's say 1.8 seconds) on tanks that had (at the very most) 10,000 hit points. It only gets harder from there. I think this is one of the only expansions bosses in the history of MMOs to remain undefeated until after the next expansion had been released nearly 7 months later.