>the most iconic fps one-man-army hero
No. Don't even go there. OP has the right of it; DOOMGUY is the most iconic. Starring in the first commercially prominent FPS tends to do that for you. "Generic games", indeed. MC, Baird, Shepard, and Noble Six are fairly recent icons. Baird is gonna get relegated to tech-bitch, poor bastard.
I recall /v/ and /tg/ going back and forth over the matter of which protagonists would actually survive the 40k setting, and finally settling on Doomguy, who's basically one badass Guardsman sans tactics, and Samus Aran, who's a walking arsenal.
Going through this thread, other iconic protagonists I can agree with include Katarn, Soldier G65434-2 (aka SS2 protag), and Denton.
OP: if you haven't, you really need to play through Doom I&II asap. The man is walking death incarnate, and unlike MC, Shepard, and N6, he doesn't have the benefit of artificial enhancements or psychic prowess... yet he still manages to be more lethal than any of them. The books/comic don't do this as much justice as the games do (don't play 3).