George Lucas said himself Darth Vader having about 80% of the power that the Emperor had. It was because Anakin was in his ruined state after losing some of his body parts in the duel with Obi-Wan on Mustafar. He was never able to reach his full potential as the Chosen One. Midi-chlorians were only present in the living things. He could have been more powerful than Palpatine or Mace Windu. According to George Lucas Luke Skywalker become finally the most powerful Force-user of all time, because he was just what Anakin could have been.
Palpatine was exteremely powerful but Mace Windu was able to beat him. In original Canon Darth Vader would not have chance against Windu, when he was weaker than Palpatine. But in Star Wars Jedi: Fallen Order which is a Canon video game, Tactical Guide states Darth Vader being the most powerful Sith in the Galaxy, so being more powerful than Palpatine. Tactical Guide says about Darth Vader "Few survive an encounter with the most powerful Sith in the Galaxy." So Vader in Fallen Order could challenge Mace Windu.