Obi-Wan Kenobi vs Darth Vader (duelling only)

All of Lucas's statements and systems are G canon, whether you like it or not, 

Never denied this, only pointed out that it's contradicted by 2-3 other forms of G-canon.

Though if we do role with the premise that certain statements have other motives not related to lore and that these motives matter in regards to their value,

Which is fact:

He couldn’t move at all, really. We had to keep modifying the suit so people could move in it. By the time we got to the first light-saber battle, we realized we weren’t going to be able to do much. And so I accepted it was an old man vs. a half-man, half-machine. But Jedi were supposed to be quite active. So for the next one, we got a really good stunt guy in, one of the best sword fighters in England. And Mark Hamill is a good sword fighter. 

So either you acknowledge that ESB Vader >>> ANH Vader as a swordsman, or that this is purely a technical reason.

then the ranking system still holds.

The 6-4-2 one is, again, unofficial and contradicted by other forms of G-Canon as well as Lucas' own acknowledgement that technical issues weren't a thing. They also hadn't fleshed out what exactly the Force could do. 

One was done for marketing reasons, the other was done for keeping the continuity consistent. Hardly the same thing.

Key word is that it was "suggested", indicating that it is unofficial.

What I find interesting is that the quote you provide does not have a lore based motive. The ranking system, however was developed to smooth out inconsistencies in the lore. Even unofficial Lucas quotes or ranking systems are G canon. Since you posited that motive matters in a quote's value, then my quote is certainly more valuable than yours. However if you really wanna run with the argument that Vader became a far better swordsman based on the actor's proficiency at sword fighting, then prepare to accept that Vader is sub TPM Kenobi.

What I find interesting is that the quote you provide does not have a lore based motive.

Huh? 

The ranking system, however was developed to smooth out inconsistencies in the lore.

...And is literally designed as a set of possibilities. It uses language such as "Ben was probably stronger than Vader" and "Maybe Ben was a six". This also opens up a whole new can of worms because by this logic Ben should have been able to dominate Vader or even just phase him with the Force, yet not once was any movement made to do so.  

Even unofficial Lucas quotes or ranking systems are G canon.

Very well, then why does an unofficial source that's problematic in itself overrule the script or film, both of which are official forms? You can also refer to From the Adventures of Luke Skywalker, which is essentially a word-for-word depiction of the film in novel form that was also credited to Lucas. All of these depict Ben as being pushed back and gradually losing the duel.

Since you posited that motive matters in a quote's value, then my quote is certainly more valuable than yours.

...No. I proved that the original quote regarding Ben and Vader was purely related to technical reasons. Yours is using an unofficial and very basic ranking system with its own flaws and inaccurate connotations to try and argue that Ben > Vader based off of one single piece of questionable G-canon as opposed to the two/three others that contradict it.

However if you really wanna run with the argument that Vader became a far better swordsman based on the actor's proficiency at sword fighting,

Now exactly what I said, but whatever.

then prepare to accept that Vader is sub TPM Kenobi.

One day this rhetoric will lead somewhere credible.

Pre ESB probably loses in Legends Canon, though by ROTJ iirc he was stated to have grown in power

Canon its pretty established Vader is second only to the Emperor in terms of power, and having fought on par with Ahsoka, beaten Old Ben who stomped Maul, beaten Eeth Koth, beaten Carbin, beaten Jocasta Nu,  I don't think he lost much skill at all.

Would be a fierce fight either way, but I think he'd probably have learned from his ROTS Knightfall self and would beat down his former mentor

Vader still loses.