This might be cheating but I’m assuming mods/DLCs to a game count as part of the game.

1. Factorio. Pretty much endless amount of stuff you can do with mods on top of a top-tier base game.
2. Minecraft, also for the mods.
3. Geometry Dash. So many user created levels and the highest skill ceiling of pretty much any game.
4. Asetto Corsa, or whatever the most realistic, moddable car racing sim there is. I don’t play this game but I don’t want that genre to die.
5. Really struggling to think of anything else so I’m just gonna go with another fun, replayable, moddable game: Balatro.