This might be my favorite puzzle game of all time. The puzzles get super hard (especially if you go for the optional ones) and you just get so many of them. The mechanics are all novel but it doesn't overwhelm you with too many of them, and the interactions between them keep you surprised until the end.

I'm also a big fan of the plot and atmosphere. The mystery slowly unravels at a pleasant pace and it never gets in the way of the gameplay. When I first played it years ago it gave me my second major existential crisis that lasted like a month so it must have been doing something right