>Why Master Chief is badass
He was genetically engineered from birth to be the pinnacle of mankind. Given the best training and the best equipment, he goes against the odds to fight for the survival of mankind.

>Why Doom Guy is badass
He's not a mere human, he's a force of nature, stuck in an eternal battle against the unending forces of hell itself. And he's winning.

>Why Prophet is badass
Through sheer willpower alone, he uploads his own consciousness into the nanosuit, merging with it's AI, and attaining life after death, all the while battling an alien force that humanity can barely comprehend.

>Why Gordon Freeman is badass
He was not forged for combat. He has no special training. In fact, he's just your average theoretical physicist, and his equipment is quite standard for the work he's doing. But when his life is on the line, he rises to the occasion, and if necessary, he will destroy a tank with just his lucky crowbar.