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.
Guild Wars was one of the only MMO's I didn't hate. It was a unique world, with cool as hell classes like Paragon in my favorite expansion. Then 2 just turned into standard high fantasy. A beautiful world to be sure, but I liked the flavor of the old one.