I'm not sure I could even pick a "most" public location. Public comes in so many flavors, and I very much enjoy tasting them... yes, indeed, a long-time aficionado of the public sex.
Probably "most" public by the usual standards would be any of the several parks in San Francisco where I've done it many many times, either in the bushes (sometimes not far off a trail) or in a bathroom. Glory days!
Bathrooms here, there, and everywhere - highway rest stops, college campuses, Sears, you name it.
A graveyard once. An elevator, which doesn't count because no one else was in it (and we stopped it). Behind a waterfall at a popular skinny dipping site on a creek. Several times in the woods in rural mountainous locations (extremely public if anyone had been there to see it, which they weren't).
And of course, lots of times at bathhouses and sex clubs wherever I've been pleased to visit them. Mostly SF area, but also DC, Toronto, Montreal, Denver, DC, Amsterdam, Basel... Not sure whether I missed any cities there, but I probably did. Definitely want to try Pittsburgh (currently my closest option), Chicago, Seattle... but recommendations taken with pleasure (message me!)