Playing through Dead Space has inspired me to make this topic. Part of me wants to say Resident Evil, or Resident Evil 2. But then I remembered that 'most influential' and 'greatest' are two different things. So as of right now...I think it's Dead Space. The sense of tension and foreboding is nearly unparalleled, except for perhaps Silent Hill 2. Sure, it's more action than survival, but it's all horror all the way.