I'll get chewed out for this one, I know it....
My favorite is Resident Evil Zero. The story was great and the characters were really cool, especially Billy Coen. Rebecca is cool, too, but she has an annoying voice. I s'ppose Billy makes up for that one. The bosses, too, were really cool, and the fact that there wasn't too much ammo boosted the difficulty and scare factor.
But if not RE0, I would have to say the first Resident Evil. Not the remake, although the remake scared the crap out of me, and was a really great game. I'm talking about the original Resident Evil. Sure, it had it's cheesy dialogue and graphics that could make people cringe today, but you just can't beat classics. That's the game I grew up with, and it'll have a special place in my heart as one of those unforgettable games.