Most important? Probably either The Ring or the remake of The Texas Chainsaw Massacre. Neither one of those films are particularly good (I think the remake of TCM is awful), but both established two of the biggest horror movie trends of the 2000s. Saw created the third with the whole torture-porn thing, so I guess you'd have to include that as well.
Blair Witch is an interesting one, though it really didn't seem to create anything significant in its wake. I guess it including that would have to be a personal call.
Best horror movie of the past decade? That's a whole other thing, and I'd probably have to go to either a Japanese horror movie (Audition or Pulse) or something like 28 Days Later, The Descent, Devil's Rejects.