I've played my share of Superhero RPGs since the mid-eighties and none of them compare to the elegant simplicity of The Marvel Superheroes Role-Playing Game by TSR; may they rest in peace.
As this blog will display, you are hardly limited by Marvel's stable of Superheroes, in fact, nearly any genre of play can be accomplished with this rules set. It's that flexible and completely awesome.
However, there was still room for improvement. Classic Marvel Forever, one of my favorite gaming sites, has an unofficial update to the rules that I found particularly appealing known as The Expert Edition. Also, the various houserules displayed at that site make gun toting heroes like my favorite - The Shadow - more playable without constantly losing Karma.
Go check out the links provided at Classic Marvel Forever ... you'll be glad ya did ...