It use to be Massey Ferguson , not sure now . I read in a farm magazine where Deere sales were now 60% in the U.S.,40% overseas , but in the next few years that would reverse .
No true research on this but I would lean towards New Holland or AGCO. New Holland is a combination of Ford, Case & IH, Fiat, Steiger, etc.
AGCO has so many different brands under their umbrella that I can't even start to name them all.
A few I can think of are: Allis Chalmers, Deutz, White, etc.
Now if you are talking a single name then JD or MF would potentially be the largest.