balliol s the home of ppe
christ church if you want to get ahead in the union
magdalen has a pretty active jcr so doesn't care that much about the union,and without people from your college to vote for you, you won't get elected to anything. Christ Church and Magdalen are the two most aristocratic colleges, ie. lots of public school/very rich people, so networking is a possibility, but in all honesty, those who are really keen on networking when they arrive tend not to be very good at it - its very easy to spot someone who is trying to add you to the contact list.
they are all excellent colleges, with balliol smaller and the other 2 larger and more grand, and more importantly, they are all oxford - whicever college you end up at, you will likely get quite attached to it, and you all sit the same exams at the end of the day.
Also you need to be abe to party - it takes a lot of the edge off plus is one of the times when etworking might be a very real possibility.
Magdalen's tutors are excellent - Ralph Walker runs philosophy (he may be leaving this year) is an absolute beast, one of the most intelligent men alive and pretty nice to boot. Professor Caney teaches politics, specialising in Marx and is a big dog in left wing theory, and Stewart Wood (Lord Wood) also teaches politics - he's Ed Miliband's right hand man.