This is narrower in scope than anyone else has offered, but to me...
If you stand at the corner of Anderson Hill Road and Purchase Street in the hamlet of Purchase, NY (a govermental division of Harrison) you are in the single most clustered mecca of private golf.
Within three miles of that spot in any direction but due south (where you will hit Apawamis, and Westchester soon after three miles) you have
Century - 0.7 miles
Old Oaks - 0.5 miles
Brae Burn - 1.4 miles
CC Purchase - 1.0 mile
Fairview - 2.5 miles
Tamarack - 3.0 miles
Doral Arrowood - 1.6 miles
(my unmentionable #1) - 1.5 miles
Not all of these are wonderful, they are all private, but you can learn a lot about GCA when taken as clustered group. Like a botanical garden of styles throughout the years
Westchester-Fairfield has a few great clusters like this, but this is the tightest, most voluminous grouping.
the intersection of Weaver St and Griffen Avenue in Larchmont is runner up with:
Winged Foot
Quaker Ridge
Bonnie Briar
Wykagyl
cheers
vk