The first time I read your question, I read it as, "what golf courses provide the ultimate test of golf?" After reading it again, it seems you are asking a slightly different question, more along the lines of the ultimate measure of a golf course's success? Maybe I am splitting hairs here, or don't quite get it, but since they are completely different questions with completely different answers, I will address each and hope that one of them is pertinent to this thread.
For me the ultimate test of golf has got to be classic links golf. Lots of strategy and lots of variety both within the round and from round to round. Minimal water hazards, maximum recoverability opportunities. All aspects of the game are tested. The American obsession with stroke play, fairness and perfect conditioning tends to water down the purity of the ultimate golfing test. Many courses to choose from in Scotland and Ireland. Over here, Pinehurst and Shinnecock come to mind as close to the spirit.
Reading your question more literally, "is the ultimate test for a golf course that it be fun to play yet challenging for every level of golfer?" From a golf course architects standpoint, I would say that the ultimate test is that the golf course be fun to play for every level of golfer, and that it challenges those that want to be challenged commensurate with their level of play. Basically I am agreeing with you, just elaborating a bit. I think one of the biggest challenges for an architect is to create enough challenge and interest for the more accomplished players while still creating a fun environment and modest challenges for the less accomplished players. Doing one or the other is relatively simple. Doing both is what creates excellence. The best golf courses stand the test of time because golfers of all abilities never tire of playing them, and I think that can be attributed to creating the right balance of challenge, variety and fun for all golfers. The ultimate test or measure of success is that the golf course is respected by the best players and enjoyed by all players.