Wow - There are some great ones on this list already. I would add plus one to just about the whole list... LOL
Okay, bad guys that I "enjoyed" watching:
Ray Park as Darth Maul in Star Wars Episode 1 - He is still the only guy to take on TWO JEDI and Kill the Master Jedi before getting owned!
Wes Studi as Magua in Last of the Mohicans - I HATED that dude by the time Uncas got to deliver some retribution at the end...
Wesley Snipes as Simon Phoenix in Demolition Man
Stephen Dorff as Deacon Frost in Blade
Luke Goss as Nomak in Blade II
Lance Hendricksen as Emil Fouchon in Hard Target
Dougray Scott as Sean Ambrose in Mission Impossible II
Nicholas Cage ( Or was it really John Travolta ?
) as Castor Troy, or was he Sean Archer???