I like Christian Bale for SolidSnake. With Batman Begins and Ring of Fire he proved that he can be the dark and mysterious dude with an attitude. And he wasn't bad in Equillibrium or the Machinist either.
But if Michael Biehn can pull another Aliens/Terminator/Navy Seals (crap movie, but he wasn't bad) he'd be a fine choice. I doubt it however, as his more recent efforts haven't been exactly up to par.