Ward and Crawford would be the top two if they were British, so America wins in that department. If you judged the best fighter in every division Britain would probably come out on top (Haven't looked, but can't think of many top American's in the lighter weights, Gary Russell Jr? Oscar Valdez? However, they're in Frampton, SElby and Quigg's division, so it would be close.

Top 5 Brits without giving it much thought

Brook
Frampton
DeGale
Selby
Khan

Billy Joe
Eubank Jr
J.McDonnell
Flanagan
Quigg

AJ
T.Fury
Yafai
C.Smith
Haskins

Burns
Crolla
Bellew
Haye
L.Walsh

Murray
Whyte
L.Smith
S.Smith
L.Williams