If it’s a PbtA please expand on your system of choice!
I’ll start:
I really enjoyed Sexy Battle Wizards, but I think The Witch is Dead speaks to me more. I like player characters just being little weak dudes and struggling against normal stuff. (both of these are free by grant howitt and 1 page of rules)
I also enjoyed rude detectives, it has a surprisingly juicy dice system for a game that’s just 4-5 pages. It’s themed around child detective stories.
PbtA I like magpies stuff, like avatar and root, but my favorite until now was Fellowship 2e I think. Just so extremely versatile, and it’s a joy to give a lot of narrative control to players.
Last one is kinda cheating: my current favorite game is rules light within it’s genre. 13th Age is the easiest ‘dnd’ I’ve ever seen, and the amount of dumb rules and bookkeeping it prunes has reinvigorated 20f games for me! But compared to other games, it’s still a bunch of rules of course.
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.