Michael- it's so tough to just pick a top 5- obviously some awesome courses then become second tier. A lot of it just depends on your perception or perhaps your individual experience at that course.
For me- Southern Hills, Pinehurst #2, Olympic, Riviera are right there, and Oakland Hills, The Country Club and Garden City not far behind. I also loved Interlachen, St. Louis, Canterbury and Myopia Hunt (have yet to play Chicago).
One thing's for sure: Open courses are for the most part an incredible treat and feast for the senses.