Voting by the quality of the modules that featured them:
Acererak 21
Asmodeus 23
Demogorgon 23
Graz'zt 11
Iggwilv 9
Iuz 24
Lord Soth 15 - the DL series dragged on for 14 lousy modules without ever being better than 'poor'
Tharizdun 24 - The outstanding WG4 just hinted at how bad Tharizdun was - that's all that was needed. This guy made the Abyss and all the Gods, good and evil, united to stop him.
Tiamat 17
Vlaakith 1
Zargon 1