Try Google. I did a little searching but didn't find what your looking for. I don't know if it is possible to keep accurate stats of the most popular p2p downloads, but you can be pretty sure that whatever is popular in the stores or at the cinema is the most popular on p2p.
Not really related, but here's a couple sites of interest.
http://video.google.com/videoranking?hl=en http://www.web100.com/