I think the question should be asked what is the Weakest Ancient Sith that can beat Maul. Mauls mastery of Dathomiri spells was shown on Rebels to be top notch, he was able to unleash Nightsister spirits, learn of Obi same location (from star systems away), and had glowing green eyes. Clearly this makes him on par with Karness Muur, except better because he can now stalemate a possessed Kanan. Maul is truly an awesome sorcerer
Originally posted by EmperordmbIt wasn't in reference to potential. Vader's power was still growing as of ANH-ROTJ.
He also said "maybe 80%" and it was in reference to potential.
Even Sidious thought Vader could eventually defeat him, because he wanted to continue the lineage of the Sith.
Which is why Sidious offered him a better suit that wouldn't restrict him and allow him to reach his full potential.