I think that there are many top stables in the United States. There are larger scale operations such as North Run and Heritage which are both fantastic at training riders of all levels in all three disciplines (hunters, equitation and jumpers). Then some of the top riders have their own training businesses alongside their own riding career, such as Kent, Laura, Mclain and many others. I think all of them have proved themselves many times in being able to turn out top riders and horses through their systems.
Thanks for the question!