I am a HUGE KT fan, but yet i find it hard to call him defintely the greatest 140 fighter of all time. I think the definite top 3 are KT, JCC and Pryor (maybe Hatton coulde join them if he returns to 140 and unifies.)
Kostya's opponents werent the greatest, but they werent all duds either, and in his prime around 1998-2002 Kostya absolutely destroyed some good names. I think Kostya would have beaten all the opponents that JCC and Pryor faced as well.

The one thing missing in his career is a win or even a close fight against a huge name like Mayweather, Mosely, DLH etc.... But unfortunately none of those guys wanted anything to do with Kostya. Too big a risk for too little reward.