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.
I was one of the eople who bought into the hype. Not enough to buy the game luckily, but I figured if I just waited 5 years or so it would be great. Sucks what happened, but on the other hand the originial with the vast amount of mods available should be able to keep anyone entertained for a few decades.