Fate is probably about as freeform / fits all as you can get, and compared to other similar systems like GURPS, making the kind of alterations to make it fit Fantasy Vs apocalyptic Vs cyberpunk etc is really easy, but the system remains evocative as you chop and change it.
I'd absolutely recommend it as a one-shot system, although it's also good for campaigns too. Players are encouraged to leave some parts of their character sheet blank and fill it out in the session as they discover who they are, which I think really helps players have satisfying characters for one-shots as they don't get stuck not knowing their characters.