A really underrated RPG is the OneDice system from Cakebread & Walton. It's a ultra-light system, ideal for running one-shots. They are various versions of it available - Space, Fantasy, WWII, etc. Each one is a complete game. They're all quite cheap and easy to run.
It's called OneDice because you only need to roll one D6 to resolve anything.
I just used a OneDice book called B-Movies to run an adventure called Beatniks from Planet X. Some 1950s high school kids had to stop a mysterious meteor from turning their classmates into filthy beatniks.
To prepare for the game, I made some custom character sheets and pregens:
https://polyhedralnonsense.com/2023/08/10/onedice-b-movies-character-sheets-pregens/
Last year I also made a custom character sheet for OneDice Pulp:
https://polyhedralnonsense.com/2022/07/15/onedice-pulp-mini-review-and-custom-character-sheet-for-a-pulp-adventure-rpg/
I want there to be more OneDice.