For me, that game would definitely be Disco Elysium. I've never connected with a game as much as with that one. I'm actually reticent of playing it again for fear of it not living up to the first experience; I felt like my first playthrough was perfect, even if technically speaking it wasn't.
Other than that, I also still think about Mass Effect a fair bit.
On a side note: if you liked the investigating and "detective-ing" of Outer Wilds, then you will probably also enjoy Return of the Obra Dinn, The Forgotten City, and The Case of the Golden Idol. I'd also add Disco Elysium to that list, but be aware it's a lot more text heavy.