I just think that accomplishments and dominance are all that we really have to base all-time rankings. We can try as hard as we want to predict how these hypothetical fights would pan out, but there are way too many x-factors that determine how a fight really goes. As far as this particular matchup goes, Kostya was a master at neutralizing good movement and I think he catches Pacquiao with shots much harder than any other punch he has ever taken - eventually winning by KO. Pac would pose KT some problems because he is so light on his feet, but he would undoubtedly begin to get caught, and he is not very good at remaining composed when he is hit clean.
Pac isn't an exceptional boxer, he's more of a fighter, so KT isn't going to have to chase him around the right. The big difference is in their power. KT has much more and would end up KO'ing Pac.
Tszyu would kick Pacman's ass up and down the ring winning a comfertable UD. Pacman's defense isn't that great and he would eat a lot of Tszyu's right hands.
tszyu was one tough mofo. if pac chooses to fight like he did diaz, he has a fighting chance to win via UD. but if he fights toe to toe with kt, kt will have the upper hand.