China has basically 3 manufacturers, and they would win if there were open global numbers, tho most are going to be 15 hp tractors....
Agco wins on a global stage, counting companies with semi-public books.
If you are just looking for North American companies, you will have to restate your question, as to the units manufactured, or the total sales volume, or the total units out there, etc.
Every company has a way of skewing the numbers to put themselves on top, so you likely can have your favorite color on top no matter which one you are cheering for. ;)