Alan Rickman in any villanous role, definitely as Professor Snape from the Harry Potter movies but he might not turn out to be a truly bad guy...
Ralph Fiennes as Voldemort
Darth Vader (the James Earl Jones scary Vader, not pussy Hayden Christiansen Vader) 😉 and the Emporer
Vincent D'Onofrio in The Cell - omg, creeped out for life!!!
Christopher Walken in True Romance
Trey Parker as Kim Jong Il in Team America, World Police 😀
Michelle Pfiefer as Catwoman, Jack Nicholsen as the Joker, Jim Carrey as the Riddler in the first 3 Batman movies
Christopher Guest in The Princess Bride
Richard Lewis as Prince John and Roger Rees as the Sherriff of Rottingham in Robin Hood: Men in Tights 😀
Rick Moranis - Dark Helmet in Spaceballs 😀
The wicked witch of the west in Wizard of Oz
Gaston in Beauty and the beast, Ursula in the little mermaid, Jafar in Aladdin, the evil fairy in sleeping beauty/stepmother in Cinderella (same voice I believe), the Prince John Lion in the old animated Robin Hood ("AUUUGH! MOMMY!")
Mommy Fortuna and King Haggard in The Last Unicorn