Which RPG do you enjoy using as-is?
To be fair, I’ve only run this once so far. It was a blast. There was so little of the system, and it’s so flexible to cover pretty much anything that you might want to do with it in the game, that it just doesn’t need to be changed to run a rules-lite game of escalating cosmic horror.
Other games I use as-is are those which have strong setting-system connection, are usually quite procedural, and tend to be fairly low complexity.