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.
The game is incredible the first time. I want to replay it, but the thought of needing to play over a dozen hours of slow railroaded cutscenes/shooting galleries during the mountain prologue, and then the exceptionally long Guarma chapter, just throws me off ever wanting to replay it fully