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.
As much fun as it was building ridiculous vehicles and crucifying Koroks, it got tiresome pretty quickly. It actively dissuades me from going back to it as much as past Zeldas.