rpg

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).

Any of the 1PG games.

If it's D&D, I think the best way to go is to do a sidequest. Level 1 characters are a bit boring, and no-one wants to spend hours leveling up a new character for one session. So, a sidequest! Use an existing character and just give them a cookie cutter dungeon. But a sidequest should be a bit silly and fun- so make the PCs play someone else's character!

If it's not D&D, I've had great fun with "Oh Dang! Bigfoot Stole My Car With My Friend’s Birthday Present Inside". It's literally no prep, so if someone cancels last minute you can just pull that out, characters made in ten minutes, and you're reading the intro text.

Any of Grant Howitt's single-page RPGs, great if you just need something fun at short notice

I like to fully improvise games in Digital Shades

This is hard one because most of the one shots are just awful as they rarely include any guidance to how to run them in short of time. At least every one shot should include a guidance of how and which parts to leave out when time is running out.

So the best ones are usually systems designed for one shots without separate scenarios but assuming you are asking about those the most palatable one has been DnD 5e adventure Sarah of the Yellowcrest Manor from Candlekeep mysteries. The middle part can be pretty much skipped if the time is limited, there is at least some guidance on how to run it and the end dungeon is short and sweet.

A wild sheep chase

Everyone is John

There's only one character to play: John, who is an ordinary office worker suffering from multiple personality disorder.
Each player plays one of the personalities, with their own stats and predefined (conflicting) goals. They are free to play "characters" from any setting or era.
In situations that are stressful to John, a competing dice roll determines which player takes over.
The other players can still talk and comment, and John will hear them argue inside his head.

Alice is missing and The space between us, the latter being borderline LARP.

Both games are pretty intense with a huge bleed potential making them memorable experience.

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 always maintain that the best oneshot is the one you cobbled together last night. Get a little weird, slap some stuff together, and let the players fly off the rails.

I haven't run many true one-shots. Not short, contained, focused, pre-written adventures. Usually I'm just cobbling things together (which is why I actually really love your books). But I've always wanted to play the We Be Goblins! series.