My name probably gives that away, but it's Dead Space for sure. I've got so many fond memories of the first two games and I got totally obsessed with the lore and other content in the same universe. I've purchased all of the novels, comics, movies, etc. and I just kinda went nuts for it.
Besides Dead Space, they're all pretty good games in my opinion, though. Alan Wake and Resident Evil 2 are really up there for me as well. And, SOMA has one of the most depressing, scary environments of a video game, and honestly, the ending kinda left me feeling crushed for a few hours which doesn't happen too often when I play a game.