Just to inject a little rigor, here are the numbers we have so far in The Confidential Guide:
9 8 9 8 Royal Worlington & Newmarket
6 6 - 8 The Dunes GC, Mich. * vol. 3
7 - 6 - Whitinsville GC * vol. 3
- 7 - - Sweetens Cove
6 - - - Aetna Springs
5 6 - - Culver Military Academy * vol. 3
Note that everyone's grades for courses listed in Volume 3 may still be changed as publication is a year away. Also, my own convention is to rate 9-hole courses one grade lower than I would give them if they had 18 holes of the same quality, though I can't say if my fellow authors share that view.
Other nine-hole courses that received a 5 rating from one or more of us:
Leckford Old Course, Magallanes [Chile], Oakhurst [W.V.], Reigate Heath, Sewanee, Traigh, and Shiskine [which of course is 12 holes]
Courses mentioned above that I rated a 4:
Mulranny, Cruit Island, Petaluma, Phoenixville, Northport Point, Pelican Beach, Dixie Red Hills, Northwood [CA]