At the very top is Oxford, with a rounded mean score of 5.0 (on a 0-5 scale) and median and mode scores of 5 as well.
The other top programs (with rounded mean scores of 4.5) are UC Irvine, Michigan and Western Ontario.
98% of undergraduates and their advisors wouldn't know to recommend these programs as top choices in philosophy of physics--how could they, but for the PGR? They'd recommend Oxford, of course, which happens to be excellent in this area, but also Harvard, Yale, MIT, Stanford, which are not. But the PGR makes available to students everywhere the opinion of leading experts. It makes available the judgment of philosophers like Princeton's Hans Halvorson (who teaches at another top ten philosophy of physics program, as it happens), and Huw Price (the Bertrand Russell Professor of Philosophy at Cambridge [another top 10 program in philosophy of physics] and a Fellow of the British Academy), and Jill North (a leading young philosopher of physics at Cornell University), and Lawrence Sklar (Michigan Professor, former John Locke Lecturer at Oxford and Fellow of the American Academy of Arts & Sciences), and David Wallace (leading young philosopher of physics at Oxford), among others. (Remember: evaluators can't evaluate their own departments.)
It's slightly crazy that some people think making these assessments available to everyone somehow "harms" students and the profession. But as we've seen, some "philosophers" are slightly crazy.
UPDATE: Here's the full list (after each school's name are the median and modes):
PHILOSOPHY OF PHYSICS
Group 1 (1) (rounded mean of 5.0)
Oxford University (5, 5)
Group 2 (2-4) (rounded mean of 4.5)
University of California, Irvine (4.5, 5)
University of Michigan, Ann Arbor (4.75, 5)
University of Western Ontario (4.5, 5)
Group 3 (5-8) (rounded mean of 4.0)
Cambridge University (4, 4)
Columbia University (4, 4 & 5)
New York University (4, 3 & 5)
University of Pittsburgh (4, 4)
Group 4 (9-14) (rounded mean of 3.5)
London School of Economics (3.75, 4)
Princeton University (3.5, 3 & 3.5)
University of Arizona (4, 4)
University of California, San Diego (3.5, 3.5 & 4)
University of Minnesota, Minneapolis-St. Paul (3.5, 3.5)
University of Notre Dame (3.5, 3.5)
Group 5 (15-19) (rounded mean of 3.0)
University of Aberdeen (3.25, 2.5)
University of Bristol (2.75, 3 & 4)
University of Illinois, Chicago (3, 3 & 3.5)
University of Maryland, College Park (3, 3)
University of Sydney (3, 3)
Evaluators: Jeffrey Barrett, Hans Halvorson, Marc Lange, Barry Loewer, Jill North, John Norton, Huw Price, Lawrence Sklar, David Wallace, Christian Wuthrich.
UPDATE: A couple of readers asked how Sydney could have made the list, given that Huw Price is now at Cambridge (he was at Cambridge for purposes of the 2011 surveys already). It's important to remember that philosophy and HPS faculties at the same university are aggregated for evaluation purposes, and in addition to the philosophers of time in philosophy proper, there are philosophers of physics in HPS as well: see https://sydney.edu.au/science/hps/staff/academic.shtml.