I agree with Wrong Turn but number 2 comes very close, damn, Henry Rollins and the girl sliced in 2 in the beginning is just insanely cool.
I got back from Paranormal Activity: The Marked Ones earlier and found it surprisingly good and one of the stronger entries in the series, but I still think the original is the best. I think 2 and TMO are neck and neck for best sequel, though.
My favorite is the third... Heh.
Just finished watching all the 5 Whispering Corridors flicks and my favorite of them all is the number 5, A Blood Pledge. Yes all of them are kickass but this one is even more kick ass, sick and creepier than any other one. You all have to see this series if you still haven't, so damn good Korean Horror.