Why saikyou in case of Mihawk is called greatest by tcb scans and viz ?
But saikyo in case of Kaido and whitebeard is called strongest.
Why they never call kaido world greatest creature or wb world greatest Man
There are 2 explanations for the sword title that you are referring to.
First, its called
Dai Kengou (not Saikyo)
in chapter 0005 for the first time:
So its translated as
WGS - Greatest Swordsman first time in the manga, not Saikyo / Strongest.
Then, Oda add another explanation for the same sword title, said ''Saikyo'' here
in chapter 0050:
Which means when you translate by using Google Translate logic, it should be Strongest Swordsman, but its a dumb translation because you don't say Strongest Marksman, you say Greatest Marksman. Its a profession.
Its not like strongest man who holds a sword, thats what some fans wants to see the title like that which is wrong.
Then
VIZ still chose to use WGS / Dai Kengou in the same Info Box, where it essentially says Saikyo / Strongest:
Because they first translated as WGS / Dai Kengou in chapter 0005, they didn't want to confuse the readers as if there are 2 different titles, its the same title.
Which is why, they still called WGS / Dai Kengou later in the some recent chapters again, not Saikyo / Strongest:
Better translation is WGS / Dai Kengou, because its
first time used like that, and you don't say Strongest Marksman, you say Greatest Marksman, thats why it should be Greatest Swordsman, its the better translation.