If any boss, I'd say Daedalus from Resistance (if they have a resistance rep)
Kind-of Anticipating: God of War: Ascension, Metal Gear Rising: Revengeance, Aliens: Colonial Marines
I'm actually starting to wonder if this is the case, I just can't believe they'd get David Hayter to do voicework just for Raiden's story. Sure it'd be a little strange having a third party character as a final boss, but with his skill in CQC, I could see him being a tough, technical final boss that is really good with blocking, countering, and throws. I'm not going to get my hopes up, but I just keep imagining approaching him in the game and hearing "you think your worthy of the title of boss?" (in Big Boss voice). Damnit please be true (although I'd be happy with Polygon man as well)
But Daedalus was taken down by Nathan Hale alone. The problem I have is that EVERY single one of these characters is being rounded up for a reason. Daedalus may have the brains. But he wouldn't stand a chance against everybody alone.
That makes me think it's "The Beast" from inFAMOUS because you never kill the beast yourself. (Putting it that way was so there are no spoilers ).