I think an architect is considered great when a golfer can fall asleep at night and remember every hole in his mind and reflect with tremendous fondness the experience he or she had when playing there, and by experience I'm NOT talking how well they played. And this is also not because of some historical event (major) or significance of the course in golf's history (home course of a golf legend, etc.).
Most of Mackenzie's courses have never hosted a major, except of course AGNC. When an architect's work can garner such acclaim on a consistent basis, then surely he must be great. Mackenzie's CPC, Crystal Downs, RM, Valley Club and Pasatiempo have always flown under the radar with the casual golfer yet when one plays one of these without knowing anything at all about the architect, I have yet to meet someone who after playing one of these, did not feel they had experienced a truly great course. They don't know who Mackenzie is, but they know what they have played was special.
If you took someone who didn't know Tillinghast and had them play his lesser known courses, I think most would be smitten by them. Sure, they have heard of Winged Foot and recently Bethpage, but I feel if they were to play SFGC, Somerset Hills, Baltimore CC, Quaker Ridge or Fenway not knowing the pedigree, they would again know they had played a great course.
I think when a architect can build a few of these, he must be considered great. And I think Doak has shown that he has, along with C&C. I think in a few years Hanse could be as well.