The problem with this exercise is that it becomes arbitrary to decide what counts as a 9-hole course vs. what doesn't.
For instance, Otter Creek is one of the best days of golf available within a 90 minute drive of my home. An all-day rate of under $100, and 27 holes of golf spread over three nines, two of which I rate very highly.
The North 9 and West 9 comprise the original 18 holes, with the East 9 added at a later date. But the facility doesn't really count the East 9 as its own course, and the North and West as an 18 hole course. It treats all three nines equally.
The North 9 would be one of the best 9 hole courses in the US as a standalone nine. As would the West. I think of those nines as their own 18 hole course, but that's only because I understand the basic history of the property. The club no longer treats those nines as an 18 hole course, but as two of the three nines that you can play in any 18 hole round.
The East, for what it's worth, just doesn't rev my engine and wouldn't make my list of any top 9 hole courses.