Well, although I like Asmodeus more than any other, I voted for Acererak.
Tomb of Horrors remains one of the nastiest games ever put out by TSR/WotC. Although some have argued (probably accurately) that some of the instant death attacks were outrageous, that one game established the lost, subterranean temple as one of the best locations for a serious, brooding campaign. And, it also introduced the dreaded Demilich, Acererak. Although it didn't give him the most personality (although it did highlight that he was a sadistic bastard who enjoyed screwing with peoples minds. Good gods, the riddles!), but it did introduce one of the best monsters this side of a Dragon.
Then, Bruce R. Cordell did the Return to the Tomb of Horrors, introducing much needed background information, and revitalizing the mega-dungeon crawl. Although I wished Cordell had waited for 3ed for this fantastic sequel, I loved how he handled Acererak, giving him an interesting (if somewhat cliched) background, tremendous power, and the ability to have a massive impact on D&D.
Other favorites include the devil Strahd, Azalin, and Orcus, although I've always had a soft spot for Tiamat. I meant to include Iuz the Old but somehow forgot. Sorry!
However, I think it's high time Asmodeus and the Lords of the Nine had a huge module/mega-adventure dedicated to them. The use Steve Perrin put Dispater through in Fires of Dis was great, and Monte Cook did a superb job with Asmodeus in A Paladin in Hell. Imagine a huge adventure from 1st to 30th level in which players slowly learn that Asmodeus is behind a ploy to not only destroy their planet, but to damn all inhabitants to Hell! Oh!!!! I'm getting chills just thinking about it!