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 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.
One thing Canon Vader has over his Legends counterpart is how the armor wasn't made to limit him...
And even if it was, he re-built it himself, pretty much from a scratch. So it really no longer should be the case.
Legends obviously have more material, but at the rate Disney Vader is getting crazy feats, he should catch up soon --- if he hasn't done so already.