I heard David Normoyle talk about a classification system once that makes a lot of sense. He compared it to wine classification. I might totally botch this but it's directionally accurate.
Premier Cru: about 30 +/- courses in the world that serve as reference points for architecture. There is certainly some ambiguity here but these are the courses you have to see at least once in your life. I'd think of these as Doak 9s and 10s.
Grand Cru: 300 +/- courses in the world. These are all very good to great golf courses that are architecturally interesting but for various reasons, might not crack the top 30ish. I'd think of these as Doak 7s and 8s and some 6s.
Cru Bourgeois: ~1000 courses in the world (truth be told, I can't remember). Solid and interesting courses worthy of play. I'd think of these as Doak 4s and 5s and some 6s.
Table Wine: everything else.
This is just another presentation of the Michelin three star system, a variant of which Sean Arble uses to rate courses. IMO it is by far the best system available, but because it doesn't provide a ranking order, it doesn't get traction among those who just have to argue whether such and such course is number 17 or number 19.
Yes, I rip off Michelin ratings, but it's partially based on worth the effort of time and travel.
3* (only a few so far) = do what it takes to play if the opportunity arises
2* = (still rare) plan a serious trip around this course
1* = worth an overnight stay to play
R = worth a day trip of 5ish hours in the car (roughly equal to the time takes to play and have drinks)
r = good trip filler or if nearby
NR = not worth either the time, effort or cost.
I usually let the reader read between the lines if R, r or NR applies and assign a rating for starred courses.