For me it’s probably Age of Empires. I used to watch my father play it when I was younger, so replaying it now feels weirdly nostalgic.

Without a doubt DayZ. Many thousands of hours in, and I'm still a n00b. This game is the final form of gaming. The ultimate survival sandbox.

If we don't count shooters or MMOs, probably some flavor of Civ.

Solitaire is probably right up there though lol.

Borderlands 2 (BL2). I love the characters, the writing and the story. Handsome Jack is my favorite villain of all time. The game play mechanics feel good to me. The difficulty levels goes up with subsequent play-throughs (True Vault Hunter Mode (TVHM) and Ultimate Vault Hunter Mode (UVHM)). There's decent DLC; "Tiny Tina's Assault on Dragon Keep" was my favorite. Also, it is possible that the story of BL2 hits for me because I also played the snot out of BL1 and fell in love with the characters there who re-appear in BL2 in very well written ways. For the record, I think BL3 and BL4 are inferior to BL2 by a long shot.

Stardew Valley. The first play-through without spoilers and without knowing what I was getting into was magical. After that, the variety of things to do and different ways to play the game kept me coming back. You can just chill and take things slow, or min/max it, or try a speed run, or just focus on your farm.