UCL!!! (opinion may contain bias)
Seriously? My opinion is that Oxford is very pretty, it was something I'd dreamed since I was small, I would have put it as first choice and probably would have loved it if I'd actually gone there. I don't like the atmosphere of Imperial - too much science for someone like me and it felt a little bit intense and a little bit unfriendly (my ex goes there so I've spent a lot of time hanging round the union/Beit hall). I love UCL lots...I reckon, with hindsight, the course at UCL suits me slightly better than that in Oxford (arts and humanities SSCs/iBScs give me a bit of a breather from all the science - I'm really interested in science, but it doesn't really turn me on).
All in all...Oxford and UCL would be my favourites out of the above. All of this talk of it being less prestigious than Imperial/Oxford...it's kicking Imperial and Oxford arses in the biomedical research RAE and it has won the UoL Gold Medal 47 times in around a hundred years. Not too shabby really.
Which one produces the best doctors? I reckon there's nothing in it.