The abilities of the Sardaukar were largely attributed to the harsh environment and brutal discipline they were exposed to on the planet Salusa Secundus, the Corrino prison planet. Only the Fremen, raised in the fanatical warrior culture and merciless desert environment of Arrakis, were capable of matching the Sardaukar. Fremen typically scorned Harkonnen soldiers as cowardly and weak, but judged that the Sardaukar fought well, and even respected them to some degree.
So what makes the greatest fighter in the narrative of the Dune universe is a merciless environment.
In the book, Baron Harkonnen unwittingly scares the emperor by suggesting making Dune a prison planet like Salusa Secundus. The emperor knows that is a way to make the best fighters - Baron Harkonnen did not.
The books is actually quite explicit about what makes the Fremen such good fighters - their hard environment shapes them.
From https://dune.fandom.com/wiki/Sardaukar
So what makes the greatest fighter in the narrative of the Dune universe is a merciless environment.
In the book, Baron Harkonnen unwittingly scares the emperor by suggesting making Dune a prison planet like Salusa Secundus. The emperor knows that is a way to make the best fighters - Baron Harkonnen did not.