This is a hobby horse of mine and pedantic as hell, but I would say the original yellow box for the Marvel Superheroes RPG (1984). A lot of people think of it as the basic version of the more widely known (and loved) Advanced rules (1986), but they're quite different beasts and play at the table very differently.
I've written about it pretty extensively over at rpg.net. The TL;DR version is that the way Karma functions (and more crucially, how much of it is in circulation) in the yellow box set (compared to what it does in Advanced) really distinguishes it from most RPGs. The determining factor of your success (or your opponents' failure) is really about how much Karma you have. That is, with most "luck" or "action point" systems, the dice are still the determining factor in success. With "basic" Marvel, the dice are really simply a means of determining how much Karma is deployed in success.
This gets even more interesting in combats. Each side is basically gambling, bluffing, and calling against the others' pools of Karma, weighing if it's worth paying Karma to mitigate the other sides' successes or blow through what they have.
It's a much more complex play experience informed by pretty simple, elegant rules than it's given credit for.
I've written about it pretty extensively over at rpg.net. The TL;DR version is that the way Karma functions (and more crucially, how much of it is in circulation) in the yellow box set (compared to what it does in Advanced) really distinguishes it from most RPGs. The determining factor of your success (or your opponents' failure) is really about how much Karma you have. That is, with most "luck" or "action point" systems, the dice are still the determining factor in success. With "basic" Marvel, the dice are really simply a means of determining how much Karma is deployed in success.
This gets even more interesting in combats. Each side is basically gambling, bluffing, and calling against the others' pools of Karma, weighing if it's worth paying Karma to mitigate the other sides' successes or blow through what they have.
It's a much more complex play experience informed by pretty simple, elegant rules than it's given credit for.