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 really ought to go back and play FEAR the whole way through. I remember playing the demo for FEAR 1 over and over and being blown away by the graphics and the bullet time[^I know Max Payne did it first, but it was my first exposure to the mechanic] as well as the enemy AI. Idk if I'm remembering right, but I think the individual bullet casings would stick around? Also, there's was something really visceral about the guns in that game.
It's spooky season, so I guess now's as good a time as any to rescue it from the depths of my Steam library!