Last year this thread definitely helped me on choosing accommodation. If you want a party hall, choose JM or Rootes. I'm at Rootes, and life cannot get any better. People here are lovely, you meet plenty of people and there's always something going on around here. The fact that I live on ground floor makes everything even more lively (top floor's very quiet, which is a good thing if you want to work but want to socialise at the same time. Ground floor's where the party's at!). I took TSR's advice last year and it worked for me. It can be quite loud if you want to work quiety or want to sleep early, but it depends on which block/which floor you're on.
Otherwise, there's an on-going competition between JM and AV. I have quite a lot of friends living around there and JM is definitely more sociable than AV (no offence!)
Westwood almost has a community of its own. I've only been there twice so I can't really tell you too much about it, my friends like it there but they would much rather live closer to everything.
If I sound boastful about Rootes, it's because I do love it and my other friends living in other halls tell me they're jealous that I'm staying there.