If you dont mind REALLY slumming it, go east slope.
But york house / lancaster house aren't bad either.
Northfield and lewes court are less social imho. I lived in northfield and my flat was great but that was a rarity to be honest. I'd go for east slope and slum it. Most of the fun parties i've had were in east slope. Thing is though, wherever you live you go to parties all over, as i said i lived in northfield but often partied in lancaster house and east slope so I wouldn't stress it really, its more a case of making friends during freshers and on your course.