Ok so after reading through this and seeing what everyone has thought and said. AND having played all the games I think you should look at their achievements. For example: what have they done to be manly/hard/badass
Samus: She's a bounty hunter who goes to planets and wipes out species all the while solving puzzles and shooting a lot of pirates and doors (side note: MP doors are stupid as hell, having to wait 5 seconds standing in front of the door is stupid but I digress). She has blown up the space pirate numerous times and has the saved the galaxy from metroids, weird space meteoric monsters [from Metroid hunters for the DS] and several other doom like things. BAD-assss.
The Doom Marine: He lands on mars, takes a look around, picks up a shot gun and blasts through 500,000 enemies before landing in hell itself only to blow up a demon ( a scary cyber-demon) and then casually moving onto Satan and killing him too. Without even stopping for a piss break.
Gordon freeman: I personally don?t see this man as "bad ass" more like "misfortunate". He lands in black mesa as a physicist and then because he pushed a lawn mower into some light he had to fight for his life to escape. He didn?t have a purpose really, just sort of went along for the ride and just so happened to escape and kill stuff in the process. Plus he has no emotions which means he doesn?t really care that he?s killing badies which is why he is kind of on this list.
Marcus Fenix: "Oh la di dah I beat up some guys the same size as me, then ran from some bats, then killed a spider, then infiltrated a mansion and then killed a general" that?s it... that?s ALL he?s done. Not very impressive. But he executed (pun totally intended) it flawlessly in a bad-ass way.
Kratos: lets start from the beginning God of War 1. He killed the Hydra, jumped down its throat, and made a guy go swimming in the hydras stomach acid. Then he murdered a lot more mythological monsters by making them felicitate his blades over and over again. Then he traveled a desert, then he climbed an effin titan and wrecked a temples shit only before getting OWNED by a big white pillar. But did that stop him? NO! He climbed outta Hades grew 150x his size and murdered a god... and that was game one. Game two, pretty much same deal except he killed the colossus, a medusa, some people who control time and another god. He killed TWO gods as he was mortal. Bad ass in his entirety.
Niko Bellic: Not badass. One reason: he is allowed choices. And he uses cover. You can CHOOSE not kill everyone (though that?s a bad idea). But you do end up killing everyone anyway. He?s no pussy, but he?s not badass 'cept for that 5 o'clock shadow.
Grand finale and my pick; Master Chief: and ill tell you why. Let?s list off what he?s done eh? He was BORN the perfect soldier [BORN PEOPLE] and trained from age 6. after 30 years of fighting and killing the covenant (which considering how many he kills in the games which is about 4-5 days usually who wants to even dream about how many he murdered in 30 years). Then in his first appearance on the Xbox he shot, punched and grenade more covenant that I bet you can count. Not to mention killing them all over again because of the flood reanimating their dead corpses. Then he saved the entire universe from being destroyed single handedly [for those of you who say "he had teammates" play the last few levels, you had no useful teammates] after blowing up a priceless artifact and an entire covenant armada, he proceeded to kill over 500 covenant ships with a few Spartan pals. That?s ride he BLEW UP an entire covenant navy in about 5 seconds [Read Halo: First Strike]. after some downtime he killed more aliens and then proceeded to drop into space and fly a bomb onto a ship of their own, blowing that up too (score is now 501 ships). Then he killed a covenant leader, killed some more aliens and proceeded to take a nap on a forerunner ship [this time it WAS single-handedly]. Then after free falling from earth's orbit he landed on earth. He feel with out parachute and survived. Then he killed 5 scarabs some more covenant ship he saved the galaxy again. That?s TWICE! This machine has saved the entire galaxy and all life in it, TWICE. For gods sake. This man is immortally bad-ass and that?s why has got my vote.
EDIT: I forgot all bout Duke Nukem! He's Badass. Got the lines, the womens, the guns and the maiming of hundreds of bad guys....pretty generic dont you think? goes for Serious Sam too.