Looking just at wrestling in a vacuum, there are many people in MMA with better pedigrees.
But in terms of his particular type and style of wrestling being suited for MMA, Hughes is one of the all time greats. He controlled almost every clinch he was ever in. He had stifling positional control. He was able to develop great submissions off of his wrestling. And had some of the best ground and pound of all time.
While his stand up was never elite, He was always, always willing to take a punch in order to close the distance to get a takedown. It was in this area that perhaps the level of Hughes competition at the time was not able to come up with a good solution for him, which was why he reigned so long.
The welterweights of that era just did not simultaneously possess elite stand-up skills AND KO power. Some of his opponents had good stand-up. Some of them had KO power. Most of them did not have both. The were not able to give him enough of a reason to not close the distance regularly.