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 give them credit for giving Isaac a voice and not thoroughly ruining the game's atmosphere because of it.
Stomping around yelling FUCK and SHIT as a properly frustrated and traumatized technician was great, as was
spoiler
"FUCK YOU, AND FUCK THE MARKER!"
Yeah his cursing was really cathartic. Particularly after being abused by necromorphs that were unkillable.