Great topic! In golf it's easy to think of big numbers (e.g. how far can we hit the ball? how long can we make the course? how much can we charge for membership/greens fees? how expensive can we make a driver? how much prize money can we hand out?). The only important time we think of how little is "what's the fewest number of strokes I can take to play this course". So this is refreshing.
So I hope this won't be perceived as nit-picking but it seems appropriate to also consider least played "great" course". Ellerston, according to raters, qualifies as such. Sunnylands does not. Moreover, Sunnylands only has eight greens so an 18 hole round has one playing at least two greens three times. There's an RTJ Jr. private course on Long Island called Three Pond Farms that has only three fairways and four greens, so it requires even more repetition that Sunnylands. Tiger's outsize practice facility in Jupiter has what, three greens? I'm sure you could construct an 18 hole routing around the property but would it really count as a golf course? I also understand Sunnylands doesn't even have a professional, so calling it a golf course in the traditional sense might be a stretch.
Other highly exclusive courses: Wolf Point, Cherokee Plantation, The Institute. Somebody connected with Golf Digest obviously thinks very highly of Alotian, but the fact that it appears on only one of the four top 100 listings recently discussed on this site seems to imply a very small amount of play. There's a course (owned by the Greenbergs - think AIG insurance) on the CT/NY line called Back O'Beyond that's visible across a lake from the Richter Park course, but Richter regulars say they've never seen anyone on it. I'll finish by mentioning a perennial world top 25 course I had the privilege of playing as a high school graduation gift and I'll never forget our host member telling us the course had averaged 5 rounds per day the previous year.