You come across a cool movie, TV show, or book, and you think, Man, I want to run a tabletop game in that world. But there’s no RPG designed specifically for it. What system do you use?
Unless another system fits particularly well, I always default to Risus.
Why? The entire Risus system can be described in one paragraph. In fact, I’ll do it now:
Write down clichés or catchphrases that describe your character. You have up to 10 points to distribute among those catchphrases; more points means they’re stronger. When your character tries to do something difficult, choose an appropriate catchphrase and roll as many d6’s as points in that catchphrase, add the results, and compare to a target difficulty number. In combat, you’ll roll against an enemy’s catchphrase; whoever rolls lower temporarily loses a die in their catchphrase. Lose all the dice in a catchphrase and you lose the conflict. Dice are restored at a rate appropriate to the genre (light superheroes: all dice restored at the end of every combat; gritty noir: one die restored per day).
So, all your players can grasp Risus in about 2 minutes.
More importantly, your players can create their characters in about 5 minutes. This lets you get a concept to the table and play it.
Risus isn’t ideal for every genre, of course; it’s better for pulpier concepts where characters tend to get into direct conflicts. If you want to run an investigation game, you’ll need a GUMSHOE-style approach to ensure PCs get the clues they need. If you’re running a Cthulhu-esque game, you may want to add a simple sanity mechanic (add 1d6 to Insanity every time a character encounters something Cthulhoid; above 10 they get jittery; above 20 they go insane).
But no system is ideal for every genre. Risus at least gives you a simple framework for making your characters, jumping into a scenario, and handling conflict.