I thought the guy that played The Joker in one of the recent Batman movies (the actor that died) was a pretty convincing psychopath.
Ernest Stavro Blofeld From the James Bond movies - played by Donald Pleasence - was good too.
The fellow who played the German Officer in “Inglorious Bastards” was excellent too.
And last but not least, Patrick McGoohan as King Edward Longshanks in Braveheart. Delightfully villainous villain.
Christoph Waltz. Academy Award
Donald Pleasence was good at being bad. I first remember him being the evil Doctor in "Fantastic Voyage" who was trying to sabotage the mission. Until he got killed by a white corpuscle, that is.
Of course, most of us remember the surgeon's assistant, Raquel Welch, looking quite fetching in her wet suit!