I find it difficult to believe I'm the only one who voted for Sigma. I thought about voting for Copy X for the same reason Hypershell did, but then I had to take a look at the resumes.
Sigma has:
Very likely killed his creator, or at least attempted to do so (Maverick Hunter X)
Manipulated countless innocent reploids and watched many of them die (Vile, Zero, X, Middi, Techno, Doppler, General, Red, and nearly all the Mavericks)
Orchestrated the destruction of both the Sky Lagoon and the entire city beneath
Pointed a planet-busting weapon at Earth, attempting the elimination of both all humans and reploids
Forced the Eurasia colony into a collision course with Earth and risked planetary destruction just to turn Zero back to his original nature
Severely disrupted the cities and ecosystems of the entire planet, so much so that only reploids could remain on Earth's surface
The way I see it, only Weil comes close to the level of destruction and manipulation Sigma has achieved in his career. But even Weil never conceived (nor achieved) planet-wide destruction.