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.
Yeah I can sign off on this. As the series went on it became clear that Bioware wasn't interested in a lot of the things that made ME1 such a breath of fresh air for video game sci fi and rpgs, and by the time you get to ME3 it's like you might as well be playing a Star Wars game.