Ben Kenobi is weaker than Obi-Wan per AGtWotF:
Vader held the upper hand in the From A Certain Point of View novel. However Vader being the superior fighter and Force-user comes purely from Ben’s POV after dying and becoming a Force Ghost. He feels anger and hatred while recollecting his past tales and isn’t mentally in a good place. It doesn’t even match what Ben thought at the time [no fear and being calm]. Vader's superiority there might have come from Ben's hyper-inflated perspective.
DK's Star Wars Made Easy states that
Ben Kenobi is the superior fighter and actually had the advantage over Vader in ANH.
StarWars.com states that an early post-suit Vader was weaker than Mustafar Vader to an unknown extent and something about Sidious testing Vader's connection to the Force [sadly I don't have the scan]. Another source places Mustafar Obi-Wan as
equal to Mustafar Vader.
But, Master Infil'a is
equal to an early post-suit Vaderb but at the same time, Infil'a is
superior to Obi-Wan. Post-suit Vader can't be weaker than Mustafar Vader if he's superior to someone who is above Obi-Wan.