The four best 140 pound fighters in the world are in my estimation.

In order

1-Timothy Bradley USA
2-Amir Kahn UK
3-Devon Alexander USA
4-Marcos Rene Maidana Argentina

Amir Khan beat Maidana in a close fight a few months back, and I if the winner of the Timothy Bradley, Devon Alexander fight then takes on Khan . The winner of the Khan vs Bradley/Alexander bout would be considered the best 140 pound fighter in the world.