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.
Magicka 2 is the exception that proves the rule "yes, even pve games need nerfs sometimes." Being an unrestrained self-endangering idiot-god was fundamental to Magicka's charm, and reducing the player's threat to themselves and everything around them ruined that.