I picked Biohazard, only because of the entries pre-BH5. It's still my favorite series, though little by little it is turning into something I can hardly recognize. I remain hopeful because that is all I can do as a fan, though I have little faith anymore. At least I'll always have my gems to play such as Rebirth.
I feel the actual best horror series however is Project Zero. It's continually impressed each and every iteration without fail. I would say Project Zero 2 is the highlight though and stands as one of the best horror games of all time.
voted silent hill, but my favourite s.horror game is alone in the dark 4
Have it on Dreamcast, actually love that game.