It's hard to call a game that tells you to make your own fun perfect, but something has to do it perfectly, all the sandboxes in the world, all the mod encouraging games and empty plains with spawnable objects, from your minecrafts to your rollercoaster tycoons, Garry's Mod stands above the rest, the amount of unimaginable enjoyment you can get out of this game is really just unmatched, even discounting mods, which I usually for obvious reasons don't include in rating, but for a game that is just so purely designed around them, you almost have to make the exception.
Some of my favorite memories are bound to this game, exploring sandbox and thinking I saw a spirit in the corner with a friend over my shoulder and getting scared. Trolling on TTT, and taking it seriously. Playing untold hours of prophunt and sledbuild with close friends. Doing unimaginably stupid things in sandbox. Hell, this games so good I fondly remember taking hours to load into dark RP, than getting banned for the dumbest most minute thing you could imagine.