Most competitive Cambridge Undergrad courses? Watch
And someone with the potential to study at Cambridge should have a) the research skills to see this has been done to death before and b) the ability to understand that it is impossible to measure.
I have noticed that the applications to admissions ratios for many of the top universities (such as Cambridge, Oxford and Imperial) for computer science are very high (I think Imperial was 10-1 or 12-1)
Normally the most competitive are graduate entry medicine and architecture in most years however. Both are very small courses with a lot of applicants, and a wider range of requirements for entry too (e.g. the portfolio for architecture, work experience for medicine).