Resident Evil 2. Love zombie movies, and RE2 was like the perfect zombie-movie like game. Great setting, creepy music and sound effects, gory, cool creatures, and a storyline straight out of an 80s horror movie. Love that game, one of my all-time top 10 games.
I can't freaking wait for the RE2 remake to happen!
On the same note, I love all the original-style RE games, but RE2 is the best, IMO.