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 cosign this hot take 100%. All these years later ME1 is the only entry in the trilogy I can replay start to finish.
Hotter take: I've never even played ME3 because ME2 was such an off putting slog.