Also here is a game I’ve tried a few times - Lasers & Feelings - it literally doesn’t require any preparation, as the players roll for the plot together. Only downside is (based on my experience) that the GM needs to have some experience and good improvisation skills to fill up the story on the spot and the figure out some additional rules, if necessary.
Furthermore, the base rules are so flexible that it can be applied to almost any settings - and the fans already made many variants (from Star Wars through Wild West to Muppet Babies). Once even I’ve made my one version based on an obscure british comedy
Finally, WAAAGH! Da Orks also worked quite well. It is not as rules light as the other games in this post, but as it is about playing with stupid ork space barbarians, you don’t really need a plot - the stupidity of the characters write their own tale