I guess I can't help but to chime in on this thread. Here's my comment on most of the countries you've mentioned so thus far.
The Netherlands: of course I know this like the back of my hand. It depends on what we are talking about. First as most of you may know The Netherlands is a tiny little place with only 16 million people but the most densely populated country in the world (or maybe #2, to lazy to google it to check now). 200 some odd golf courses. Certainly home to some of the worst courses in the world I'm guessing. After all there is nothing worse than playing golf in a Dutch Polder (reclaimed land that was under water, I call it swamp). Mind you I'm being very opinionated with this answer. Anyway these course are all perfectly flat and muddy if you simply think about rain. There are some 300,000 golfers here and while there are 200 courses there are perhaps 10 that are worth playing and arguable 5 or 6 that are great ie, world class links and heathland courses. I'm not sure if that makes NL the most underrated country or not.
Sweden on the other hand has about 600 courses and 600,000 golfers. I think there are a lot of nice courses there but most I've played seem to be kind of Parkland type or inland links type courses. The top course in the country is Bro Hof Slot where they play the Nordea Masters and while it a good course and the location is quite beautiful as well as the castle it says enough to me if they call that the best course in the country. It's basically a course set up for the European Tour and can play as long as 8300 yds which they never do of course. Lots of water.
France a few good courses and also a small number of great ones but not more than 3 or 4 in my opinion.
Out of those countries and their relative size, I'd say NL is your best bet for sure. Everything is close by to some extent accessible and a few have been designed by some of the more famous classic architects. ie, Colt.
Japan, I'm up in the air on. It's high on my list of places I'd like to go play that I've not been and I don't dare pass judgement on it until I can experience it first hand and not through the pictures. The culture is indeed fascinating as well.
South Africa may well be the most underrated golf country as well. I can't yet say unfortunately.
England, Ireland, US and Scotland certainly are not, although I think parkland courses in general may well be. Maybe someday I'll be proven otherwise.