These last 12 years haven't been my favorite. I am not huge on serious horror unless it is really good. Went to Descent in theaters and thought it was very, very suspenseful, but it wouldn't be on my list. The Ring, Final Destination, Saw are just nothing special to me. Hills Have Eyes remake and Wrong Turn. I still can't get excited though I do enjoy them. Trick R Treat is definitely great.
My list is something like
Cabin in the Woods
28 Days Later
I would say the best horror film since 2000 is either Grindhouse or Cabin in the Woods.