Ajunta Pall would be able to defeat Karness Muur. Pall was better than Muur when it came to dueling. Muur focused on force power while Pall was more skilled in dueling. Muur was proficient at using a lightsaber, but Pall was even more proficient at using his blade. Also, many Sith were out to get Pall’s blade after his death. It was stated to be more powerful in the dark side, which we can predict that it may have given the user an upper hand somehow. Muur was probably a more powerful force user though. He was able to whip up force storms even after his death, which a slightly doubt that Pall can do. Muur’s amulet wouldn’t be much of a help in the battle with Pall because it controlled leviathans, and Muur couldn’t just make one so fast or Pall would slice him. Pall also had powerful skills in the force, so he can at least fend off lots of Muur’s attacks.
Pall was the leader of the Exiles, and the leader of the Dark Jedi when they lost the Battle of Corbos. The Book of Sith states that he personally killed the reigning Sith king and took his place. He was the strongest Dark sider at that time, or else the others wouldn't submit to him.
Did one or more of his compatriots eventually surpass him? We just don't know. But we know that Muur, Dreypa, Xoxaan and Sorzus Syn became stronger and acquired many abilities by melding their knowledge with all the ancient Sith lore available at that time. It would make zero sense for Pall to be the exception.