Gul'dan.
Arguably the only major villain in Warcraft that freely chose evil.
Deathwing, Sargeras, and Loken were driven insane. (and Cho'gall was nuts to being with)
Azshara and Kael'thas didn't really have a choice but to serve the Old Gods and the Burning Legion
Arthas, Ner'zhul, Xavius, Medivh, Archimond, Kil'jaeden, the Scourge, the Old Horde, and most of the Burning Legion were corrupted.
The Old Gods and any demons not corrupted by the Legion (such as the Nathrezim) are simply evil by their nature.
Illidan, depending on how you interpret him in Burning Crusade, is either a well-intended extremist antihero, or he was driven insane/corrupted by the Skull of Gul'dan.
Malygos was a well-inteded extremist antihero.
When Ner'zhul discovered that the orcs were being used, he broke down. Gul'dan discovered the truth at the same time. He took his chance to take over and lead the orcs down a road of damnation that directly or indirectly affects every event afterwards. He views his entire species as a means to an end and thinks nothing of making them slaves to demonic influence, or taking two entire clans in search of ultimate power the night before a huge battle, costing the Horde the war.
He's evil because he wants to be, which really sets him apart from others in Warcraft.
Also, Revolver Ocelot for his complicated plans and backstabbing ability. I really want to see Konami vs. Capcom, simply to see him working with Wesker, another backstabbing fanatic.