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.
It's wild that they bungled WC3 reforged when SC Remastered was such a home run. They even managed to shake up the pro meta a bit by fixing the old sprite limit bug and making Valkyries viable, and it turns out that doesn't break the game!