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.
Its a situation where I'm not sure I can truthfully say that I like the story, though its fine, but I do really, really like the characters and their development.
But yeah I mean 4 is a much better game lol.