My money is on one of the first ones being Mountain View, CA. Besides being one of the most heavily used superchargers in the country (and the most heavily used for much of its life), it also has the distinct pleasure of being one of the most frequently broken superchargers. A few weeks ago, I was getting a comically bad charge speed (and possibly other problems, I forget), and I called to report it, and they said that only one single cord in the entire place was reading as healthy at that particular moment. So 11 of them were charging at below normal speeds because of equipment malfunctions.
And this has been my experience more often than not at that location. There was a rumor that as soon as the Sunnyvale and Cupertino superchargers came online, they were going to rip out Mountain View and rewire it. I'd imagine they were waiting for the new v3 hardware.