Michael Biehn. He's incredibly underrated, which is a shame because he's an awesome actor. It's funny that he's starred in some of the biggest Sci-Fi films of all-time, like The Terminator, Aliens and The Abyss, but he never went on to become a major star like he should have. He's still working, but netherless very underrated.
I like how James Cameron casts (and considers) him quite frequently in his films.