I live in New York City so a lot of great options within 50 miles.
Guilty of being a homer, but my favorite course is Wykagyl. Classic. Architect: Lawrence van Etten, with changes by Ross, Tillinghast, and more recently, Coore & Crenshaw. Very challenging without kicking you in the teeth (or elsewhere), lot of elevation changes, interesting greens, cool bunkering, every hole unique, somewhat unusual 5 par 5 and 5 par 3 layout.
That said, if I had to look beyond my home course, I might call "most underrated" Plainfield Country Club, which I had the pleasure of playing this past weekend. Classic. Architect: Ross. Ribbon-like fairways that reminded me of Crystal Downs, elevation changes and snaking fairways that reminded me of National, and amazing green complexes, many with huge false fronts and "backstops". The finish is simply outstanding. I knew the course was supposed to be very good, but was surprised how interesting and full of quirk it was. I haven't played a ton of top 100 world courses, but based on the ones I have played Plainfield strikes me as being easily on the list, and probably closer to 50 than 100.