The Hispanic Society museum has everything you could ask of a small museum.
The collection features masterworks from great artists. Its focus is broad enough to be of interest to the general public but narrow enough that you really learn something about a specific topic -- in this case the evolution of Spanish painting and decorative arts. It even has a beautiful building to house the collection and show it off to the public.
Yet nobody comes, as you can probably guess from the fact that I'm the only person other than an editor to review the museum, which has been around for the better part of a century.
Individual visitors benefit from the relative obscurity. I had the museum nearly to myself on a recent visit. But as much as I enjoyed the peace, I felt bad that this great little museum doesn't attract more people.
It's not as great as the Frick, but most people would enjoy this place far more than MOMA or the Guggenheim.
Pros: First-class work from great artists
Cons: Hard to get to