Not necessarily.
In terms of raw power with the Force, Maul could have matched Vader. In terms of harnessing the Force for subtle things like mind sway, premonition, force lightening, large scale battle and combat mediation and influence, etc... he simply could not focus the Force that narrowly due to the over whelming power of it that he possessed. He basically operated the Force like a shotgun instead of a sniper rifle like other Force masters could do.
He was a blunt instrument -- a "fighting animal," as Darth Tyranus said -- but I sincerely doubt he could've matched Vader in sheer Force ... uhh, ability
Anakin was supposed to become "more powerful than either of us," or so said Sidious of himself and Yoda, and Sidious completely and utterly
p0wned Maul. Did you see that Clone Wars episode in which he effortlessly killed Oppress and bitch-slapped Maul simultaneously?
I do think that at his best, Maul would've given Vader-in-the-suit a hard time in a lightsaber duel, however.
The most powerful Sith of all time has to be Darth Marek. This dude was standing on a planet and used the Force to grab a big ass Star Destroyer that was in orbit and rip it out of the sky and bring it crashing to the planet. He even beat Vader twice and almost defeated the Emperor.
That's very true, but all of the Force Unleashed stuff is now deemed apocryphal. Even before Disney redefined the canon/official policy, I'm pretty sure TFU was regarded as a "Legends" story. Besides, fun as that game was (I never played pt. 2), Marek beating Vader and Palps was just stupid. The kid was a total Gary Stu.
Darth Malgus was pretty damn powerful too. He led the sack on the Jedi Temple and killed dozens of Jedi Masters. He also was able to stop a ship in blast off and hold it suspended in the air despite it's turbo and thrusters going full blast. He reached the point where he no longer simply used the Force, but rather he WAS the Force. He was virtually unstoppable.
But didn't a Stormtrooper/Republic trooper precursor beat on him pretty good? I know he was fighting Jedi in the same battle, but still ...
On a side note, the Sith lords of old were much more powerful than Vader or the ones today. Even more powerful than the Emperor. The current Siths are watered down and weak which is a flaw in their Rule of Two mindset. A master teaches the apprentice all he knows, but not everything the master knows. So when the apprentice finally rises up and kills the master, all the knowledge that the Master has not taught yet dies with him. The apprentice becomes the master and the cycle repeats itself. This is why the old sith lords could to unbelievable feats with the Force.
Well, again, even before the canon policy changed, many sources explicitly stated that Palpatine was the most powerful Sith Lord,
ever. His master was literally afraid when he looked at Palpatine's "aura," noting that it was so dark, Palps was basically the nexus of the Dark Side.
You do make a good point about the Rule of Two, but I also have to figure that, for all of the knowledge that might be lost in the process (though that's what Holocrons are for),
new knowledge would be gained over the eons as well. Think of it as 3 steps foreward, one back (if that much).
Regardless, Disney's policy pretty much wet blankets any such discussions; as I said, the newest material reaffirms other, older sources that established Sidious' supremacy over the Revan, Bane, Nihilus, Annedus, et al.
