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.
honestly i couldn't even finish DS3 it because it crashed three times in the same place and on the third time it deleted my gun and i just went "nah lol" and peaced out