I agree that Saw as a series is pretty good. You just have to place yourselves in the shoes of one of Jigsaw's victims and it's scary as all hell.

BUT!!! How can you not go with the Texas Chainsaw Massacre? Did you see the original? It is the scariest, most brutal film I've ever seen, and completely believable. That crap could happen, and what's even more creepy, is it probably has.