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.
There was so much I loved with New Dawn, but then they padded out the gameplay to MMO levels of grinding and refused to add a NG+ to play with all the superpowers. Shame, I actually liked the basebuilding, but running oil back and forth sucked.