Nastiest... hum... Yami no Bakura wins on the normal use of the word, "nasty," due to his eating habits.
'course, as far as attitude goes, Kaiba wins. He always said things as they were, and in the most cutting way possible. The simple fact along with it was that he was right. Yami no Mariku pulled some of the arguably most evil stunts, but he didn't have the ruthless aspect to him that Kaiba did. Yami no Bakura had a ruthlessness to him, but he always kept his own goals in mind, rather than taking every opportunity to verbally abuse the Yugi-tatchi. Mind, I do think Yami no Bakura was a better villain than Kaiba (even including the non-butchered Japanese version of Kaiba), but Kaiba had the best attitude and personality.