Any of the 1PG games.

With a GM? Jailbreak for Unknown Armies, hands down. Needs to have 5, best 9 people for the best experience. There is a newly released freely downloadable version made converted to another system on DTRPG afaik. GMless? Probably Alice is Missing, though I really like 10 Days Without Sunlight (as long as the scoring mechanic is ignored).

I love the level-0 “funnels” from Goodman Games. If I have to pick one, let’s say the classic, Sailors on the Starless Sea.

They’re easy to pitch, and really help establish a tone, especially for players who bring a lot of preconceptions from 5e.

I like to fully improvise games in Digital Shades

“Chariot of the Gods” for the Alien RPG is on the top of my list.