I'm another Oxford grad, but I have also worked at the University, and several of the colleges, and am currently in the College office of one of those that does not offer classics! I'd back up the point made by so many pps that many new undergrads are placed at colleges other than the one they applied to - the pool system is increasingly important. So don't get too attached to a particular one!
Equally, the Northington table isn't a great indicator of how an individual student is likely to fare, simply because different colleges use different methods to encourage good results. Some apply a lot of academic pressure, which works for some students, but can be a little problematic for those who are less resilient. Other colleges can successfully use a heavily involved and nurturing environment to encourage good work, which again works for some but is annoying for others.
And while all of us alumni can tell you what our colleges were like when we were there, colleges change character with each new cohort, and especially with changes in staff. That's what working in Oxford has really shown me - that because college communities are small, their character really is influenced by small changes, and what a college was like 5 years ago, or 10 years ago, isn't always very useful.
None of which is very helpful, sorry! I'd agree with using the quiz on the Uni Admissions site, and then reading the Alternative Prospectus from Oxford SU (no longer OUSU!). Definitely take the info about number of applicants into account, and then visit the top few.
And I'll add my faves - because why not? - are currently Univ and Worcester. And that Teddy Hall remains very posh!