First of all it is great that you are considering Cambridge!
As has been said already, there are 31 colleges, and hence, there is a great deal of variety. You have to decide not only whether you would prefer a more modern or a more traditional setting, or upon a college's proximity to the more bustling centre - though the distances are relatively minuscule (apart from Girton which is about a ten-minute bike's ride away) - but you may also want to consider size. If you are looking for a more 'intimate' experience, in a more close-knit environment, then obviously one of the smaller colleges such as Corpus Christi (with just 250 undergraduates) would suit you more than a larger college such as Trinity (with 663).
It really depends on you. For that reason, I am steering clear of any attempt to recommend "this college" or "that". I think it is important that you make the right decision as suits yourself, and that it is wrong of anyone to make it for you. Besides, the course that you will be studying is a universal course within the university, and is not likely to differ much between the colleges anyway.
When I chose my college, I read through prospectuses for each college (it was also useful to read the Alternative Prospectus, written by students, containing honest opinions about various aspects such as food, accommodation and facilities etc.) and I decided upon about five or six colleges that I liked the sound of. Then - the importance of which I cannot stress enough - I visited them. I had settled before my arrival upon Clare College, and though I really liked it, when I visited Jesus College, I absolutely fell in love with it. It was friendly, relaxed and had a good, unpretentious mix of tradition and modernity. Indeed, it shows how much a visit to a place, and how much you warm to it can make the decision almost involuntary.
Finally, as I am sure you will have heard enough, Cambridge is a great university, and the vast majority of people end up being happy and having a great time at whichever college they should happen to have ended up in.
I hope that helps, and good luck! I should also love to hear which one you have chosen!