I've been to the Kubota factory outside Atlanta. BX, TG, T, G, GR and the RTV are made in the USA. The engines and tranny's are done in Japan, the rest is all done here. All of the loaders, backhoes and mower decks are done here by KMA. Many of the implements, as well as the smaller tractors are actually exported to other countries. All this is done by KMA - Kubota Manufacturing of America, am american company that employs about 4000 people, all of which are non-union. A few miles away is the final assembly plant for all the larger equipment that dealers don't setup. The KX construction line is in the process of being moved to GA as well, that building will be done any time.
The tour was amazing, I've been on a few factory tours and it was the most advanced I have ever seen. I know people who have been to the plants in Japan and they say they are several times more impressive than this was.
I have not been to NH's plant, they do alot more outsourcing so I am not sure how much there is to see. They do have several final assembly locations for the tractors, I am pretty positive the company itself is not involved in any actual implement production.
To my knowledge the "most american" tractor would be the Cub Cadet 5000 series.
That tour was amazing!