I agree with Darth Thor here. Regarding the powerscaling in the respective universes, Canon Vader places much higher as far as Canon powerscaling than Legends Vader places by Legends powerscaling.
That being said, I'm pretty sure if we took all of the feats and scaling of Legends Vader and compared them to Canon Vader, Legends Vader comes out on top. This is because Legends feats and scaling were waaaaay more absurd than Canon. The high end feats on Legends fit better in DC or DBZ than in Star Wars, and as a result, Legends scaling pretty much always results in Legends characters being far greater than their respective Canon counterparts.
So, respective, in-universe powerscaling, Canon Vader>Legends Vader. Actual feats and conventional scaling, Legends Vader>Canon Vader.
In terms of feats canon Vader is better. Legends has better scaling though such as TFU games for example
__________________ "Vader's pulse and breathing were machine-regulated, so they could not quicken; but something in his chest became more electric around his meetings with the Emperor; he could not say how. A feeling of fullness, of power, of dark and demon mastery -- of secret lusts, unrestrained passion, wild submission -- all these things were in Vader's heart as he neared his Emperor. These things and more."
That’s more of an impressive showing for Maul to be fair, and it was ANH Vader, who is meant to be a shadow of his former self (RotS Anakin). That Vader would beaten by Grievous, Anakin, Obi-Wan, Mace and maybe even Dooku in a sword fight.
In terms of rankings, Canon Vader's defenitely higher, in his continuity. If the two were to go in a head to head brawl though, I'm thinking Legends Vader would win.
In Legends, he was above average, but there were many force users more powerful than him.
In Canon, he's one of the most powerful sith to ever live, which is obvious when he easily overpowers every threat that he encounters, including ancient force users like Mommin and Rur.
But then again, in Legends, he was competing with people who could destroy planets and cause supernovas and create/move black holes. Surviving Legends Palpatine's force lightning in RotJ alone makes him more powerful than Canon Vader.