Hades. I literally cannot think of one thing I'd change about it

I read the title and got halfway through the first lil paragraph and the word “Portal” immediately popped into my head lol.

Otherwise, I think Titanfall 2 has the perfect FPS campaign. Just like Portal, it’s fun, replayable, fat free, and every level is exciting and finds a way to surprise you. The story is far from original but it’s told so well because of the fact that it’s an interactive video game and the two main characters are supposed to be the player and a robot so their relationship is defined essentially like the player’s relationship to the game. It’s not as approachable as Portal since the controls have more depth and complexity, but that’s why I said it’s the perfect FPS campaign.

This is probably going to sound dumb because it has so many variants, but Tetris.

It's simple in concept while being highly addicting, with an easy to understand goal. You can teach someone everything they need to know in under 30 seconds. Even the most stripped down, basic version is still engaging.

Few games, even ones of equal simplicity, can attain that perfect balance.

Return of the Obra Dinn

Phantom Liberty is pretty high up there IMO