It depends how you look at it.
If you look at the all cargo operators, it is Fedex.
(other carriers can carry more freight, but they use their passenger flights as well, and in this category, FDX should rank between 3rd and 6th)
In terms of number of aircraft, once again, it is FDX
Ecoute les orgues, Elles jouent pour toi...C'est le requiem pour un con