One-shots work well with pretty much any cliché storyline that the players can swiftly get the grasp of and jump into without too much time spent on exposition or build-up.
> "There is [Evil character] doing [evil stuff] in [place], please go and defeat them."
The evil character could be a tyrant overtaxing the population, a ghost tormenting a small rural town, a lich in the nearby dungeon doing shady stuff, a dragon who kidnapped a princess, etc... Pretty much anything that the players may have familiarity with and without arguing too much about the best course of action.
Depending on the premise, you could lead the players straight to the dungeon, or let them investigate a bit and come up with a plan to infiltrate the evil character's lair. Whatever you choose, keep it short and easy to do: one-shots have very little time to dwell on specific details or plot beats, and if your players waste too much time on the planning/investigation phase, there won't be time left to actually fight the bad guy.
Even if it's a one-shot that won't ever have a follow-up, including loot for the players to find is always an exciting addition. Players, like crows, love shiny thingies
