Tszyu is highly regarded by all and respected universally.
At 140lbs his power was hard to equal and he trained hard to accompany that natural gift with stamina speed and boxing ability. His case for being thee best 140lber of all time is a strong one and perhapsI wouldn't hesittae so much in granting him this staus if it were not for the great level of competition at this weight.
Julio Caesar Chavez was great at this weight. Aaron Pryor had a short brutal reign exclusively at 140. Roberto Duran resided here long enough to make his case. Gavin Rees....just kidding!

For me Tszyu very well may be the best 140lb fighter of all time, but until I study his numerous competitors thoroughly first I hesitate to brand him the best.