I appreciate there's over 60 colleges that make up Oxbridge, however it's very hard to rank them in beauty, as students probably have only attended either Oxford or Cambridge, and even then have not seen or appreciated the beauty of all or most colleges on offer. Furthermore, most students have not even attended Oxbridge. Therefore, the only way for them to perceive beauty is through carefully photographed images, which may not be representative of beauty, as you can quite easily hand-pick the ones YOU find beautiful, and also filter out the negatives which can exist in these colleges.
Henceforth, I doubt this will be conclusive or unbiased, and may really be seen as what TSR students, whom don't necessarily attend any of these colleges, think of their beauty based on images the OP picked, of only 6 available options (which happen to be some of the most popular or largest colleges).