This is a very subjective question, since every individual will have different experiences and different goals in their field. One 2012 ranking lists Cambridge, Oxford, Edinburgh, and the London University College in the top 4, while a 2011 study by Timeshighereducation.co.uk listed Harvard in first place worldwide, with a 100% reputation; only MIT listed close, with an 85% reputation. The answer to this question will also depend on finances; a scholarship student will have an easier time getting into Harvard or Cambridge than a student with a median GPA. In the end, the selection of a university in the medical field in the UK will depend more on finances than anything else; Greenwich university has a reputation for being cheap, but also for "teaching to the test," and not necessarily imparting useful knowledge. For the best degree and highest prestige, any of the top five universities listed above should be suitable.
There are many reputable medical universities in the United Kingdom. In asking about medical universities it is my understanding that one is looking into universities that prepare one to become a physician. Since the medical field is broad ranging from nursing to dentistry and research, I have listed those universities that are relevant to the medical field in general.
The University of Aberdeen, http://www.abdn.ac.uk/medicine-dentistry/, provides education in dentistry, medical research, applied sciences, and applied medicine.
Queen’s University of Belfast, http://www.qub.ac.uk/, has educational opportunities in biological sciences, research, chemistry, dentistry, midwifery, and psychology.
The University of Dundee, http://www.dundee.ac.uk/medschool/, offers education in medical, nursing, and dentistry.
Other medical schools include but are not limited to:
Royal Free University Medical School, University College London Department of Medicine, Guy's, King's and St Thomas' School of Medicine, St. George's Hospital Medical School London, St. Bartholomew's and Royal London School of Medicine & Dentistry, Imperial College School of Medicine, London, London School of Hygiene & Tropical Medicine.
An additional resource for information is The Association for the Study of Medical Education, UK. Medical Education, UK. They will be able to provide a rating for the colleges as well as more detailed information.