I'm going to go out on a limb made solidly of NYC pride and say that the Union Square Greenmarket is the best farmers' market in the United States.
Granted, I'm guessing here, having not been to farmers' markets in every state, but once July rolls into August, September and October I just can't fathom how a produce selection gets much more amazing than what's for sale on Monday, Wednesday, Friday, and Saturday here. (Well at least in a temperate climate... I've been to some pretty amazing markets in Central America, India, and Southeast Asia that are hugely impressive.)
Here's a taste of some of the bounty on display now.