THE WARCRAFT LORE KEEPER IS HERE! : D
Gul'dan was the one who helped Medivh open the Dark Portal, from the Draenor side. He was the first Warlock, and leader of the orc clans, until Doomhammer smacked him down, that is.
Afterward, he went to the Tomb of Sageras (Medivh promised to show him it if he helped with the Portal) searching for Sageras's body (Which was, in fact, an avatar of Sageras. The REAL Sageras would obliterate the entire world if he were to pass through.) and the power within it. Sageras's demons still inhabited the tomb, and quickly slaughtered Gul'dan and his orcs. Gul'dan's skull was used as a demonic artifact afterward, and its the same artifact that Illidan used to become half demon, half night elf.
And the original Lich King was Ner'zhul. After Arthas got to Icecrown and broke the Lich King's prison, he became one with Ner'zhul.
And, I disagree with Doc actually. To me, Gul'dan was very...boring. He had an important role in the books and Warcraft 1/2, but he was simply killed in WC3, barely mentioned. My choice would either be the Old Gods, Neltharion, or Kil'Jaeden.
The Old Gods are manipulative f*cks, and don't even exist in the physical world, yet they caused almost EVERYTHING in the warcraft universe. They had a hand in Sageras's betrayal, they corrupted Neltharion, the first causing the creation of the Burning Legion, the second causing the almost extinction of the dragons, which subsequently caused Malygos to wage the Nexus War, etc.
Neltharion, because he is completely insane and crafty. The most recent Warcraft book proves this...but I won't spoil it.
And finally, Kil'Jaeden, who first made the Orcs what they were. He corrupted Ner'zhul, the original leader of the Orcs, and ended up making him the Lich King. Kil'Jaeden was never seen in actual battle, unlike Archimonde, and yet he had so much influence on everyone...
But ya, Warcraft lore has TONS of awesome villains