As title is, just give a description why.
I would go with Gemini Man... honestly, you thought fighting one at a time was enough, take quick man and split him in two. On top of it, make him faster and harder to kill. It's just nasty, and I still till this day can't beat him without his weakness. Looks cool, hard to beat, two for one, and has a two nifty weapons (buster and laser).
Note: Nitemare is the cause, he found me the Mega Man collection for PS2... xp
