The original Resident Evil on Playstation. I would have said Resident Evil 2 for the longest time, myself. I'd still say 2 is the superior game, but perhaps nostalgia set in and I look back more fondly now upon the original. I can remember just about every area and scenario from RE 1, it was just so memorable to me.
Though I regard it highly, I've never been a big fan of RE 4.