For me it's Diablo II. Granted I've played my fair share of D2 since launch, and also recently on a private server with a comrade from hexbear, but I still feel like years later the game didn't grab me as much as D1 did.
Granted I don't hate D2, but for a game that I keep coming back to, D1 takes the prize.
It's such a fucking shame that the discourse around TOTK is completely dominated by stupid shortsighted "70$ DLC" arguments. TOTK is one of the most exciting sequels I have ever played. It's everything people want when they buy a new console generation, except on the same old hardware. The toolset the game gives you to solve puzzles is absolutely insane, ascend and recall alone are genius ideas and they all fit together absolutely perfectly. They could not have made that game had they also made a new map and I am so glad they didn't.