I was playing around with the FADDS aviation database and came up with a report that shows the top 1000 airports in the US as ordered by number of general aviation operations a day. The FAA source data is not entirely accurate but gives a general idea of activity.
No huge surprises: the biggest GA airports are near LA, Phoenix, Daytona Beach, and Minneapolis. A lot of these ops are probably training related; every single touch-and-go practice landing gets logged. I was surprised my little home airport of San Carlos is #71 at 350 ops / day. I have a hard time believing that number is really correct, that's roughly 30 ops / hour of daylight.