It's a big question but I suppose King's and John's are on one tier of awesomeness (as they perform so much more than the others...then again, the quality of the trebles can sometimes vary more than you'd expect) and then you get Trinity and Clare on another, and then it's Caius and still others. Maybe I'm not being utterly fair to Caius, who are very good indeed, but I just never get the same blending quality I hear from Trinity and Clare...and their chapel (while lovely) is about as resonant as a cardboard box, so that's a bit of a let down.
Generally, apart from the number of services per week, you can tell by who does the most exciting tours/recordings/projects and also by the calibre of the music on the different lists. Ultimately, though, lots of choirs at Cambridge are superb- and that's definitely the reason why I applied here as opposed to Oxford...