What's the best, least-well-known course to host a major?
Not Portrush; even thought it hasn't hosted the Open in years, it's generally regarded as one of the top links in the world.
What about Pecan Valley in San Antonio? Julius Boros still holds the record as the oldest winner of a major when he won there in 1968 at age 48. A Press Maxwell design; a friend who played it several years ago said it was quite good.