I think Yami-Bakura, Dartz, and Yami-Marik are the most evil guys in Yu-Gi-Oh!, and I like Y-B's voice in the JP version of Memory World, and Dartz 'cuz of his Orichalcos cards and remarks in the dub: "You have a speech for everything, don't you, Pharaoh? You must get so sick of being right all the time.", and Yami-Marik because of his repulsiveness, and, if they kept in his sticking-his-tongue-out scenes, it would've been better.
Aside from them, there's also Naraku and Sesshomaru from Inuyasha.