At least according to The Brewers association, they are:
Widmer Brothers Brewing in Oregon heads the list, followed by Pyramid Breweries (Washington), New Belgium Brewing
(Colorado), Boulevard Brewing (Missouri), Deschutes Brewery (Oregon),
Kalamazoo Brewing (Michigan), Magic Hat Brewing (Vermont), Harpoon
Brewery (Massachusetts), Rogue Ales (Oregon) and Kona Brewery (Hawaii).
Check out the article on Realbeer.com for the details. I'm happy to see Magic Hat in that group. Brings back memories of a Vermont snowboard trip.