I wont say Mali cause its not possible to beat it right now so, Inca/Mongolia/Aztec probably cause i have not played them much.
Yup. You just cant do Mali yet.
Mongolia for me hands down.
I haven't tried the Incas more than once and Rhye said he tweaked it to make it easier.
After Mongolia id probably say Ethiopia simply because it requires some luck to hit the first goal. Last...im really not sure.
I find the USA difficult as I find it hard to kick the Euros completely out for the same reason I find Spain hard (unless you hit France and England at home). Persia is also difficult but Persia has always been difficult although I feel its even harder under BtS. In fact Persia is my 3rd choice.
Mongolia, Ethiopia and Persia.