Multiple Jedi mastered all seven forms during that era, but Mace Windu was the only master of Vaapad.
The only to master Vaapad and not fall to the Dark-Side.
Which in my opinion, says something about his will against the Dark-Side.
He didn't turn when fighting Palpatine even though, for sure he had to been overwhelmed by the power of the Dark. And thats even controversial, some think he did turn hence his wanting to kill Palps.