Marcus Fenix
Of all the marines, in all the alternate galaxies, in all the fictional universes, the gruntiest, muscliest, most trigger-happy of them all is one Mr Marcus Fenix. There might be some back story about him being imprisoned for leaving his post (in order to attempt to save his father, no less) and he might be best mates with Dominic Santiago and they might all have double-barrels of fun blasting locusts and there might be a bit of drama here and there too, but what it's all about it really is making you feel tough. Really tough.
That what Marcus Fenix does. He makes you think you're the toughest cookie in the jar, the hardest of all the nails, the brickiest of all the shit houses. Improbably proportioned, implausibly muscled and unspeakably hardcore, this is the badass COG trooper we all wish we could be. He's no Hamlet, and you don't much care what's going on inside his head, but as an ammo-guzzling baddie-blaster, he's pretty hard to beat.