This is a matter of perspective and interpretation.
I don't think that George Lucas have personally claimed that Sidious is the most powerful Sith Lord in "galactic history." He did pointed out in one of his interviews that Sidious is the most powerful individual "in the galaxy." Sidious was not only powerful in the ways of the Force but also the supreme ruler of the Galactic Empire which ruled over the galaxy.
Now keep in mind that Mr. Lucas have stated this as well:
"There are two worlds here," explained Lucas. "There's my world, which is the movies, and there's this other world that has been created, which I say is the parallel universe—the licensing world of the books, games and comic books. They don't intrude on my world, which is a select period of time, [but] they do intrude in between the movies. I don't get too involved in the parallel universe."
Mr. Lucas always represents his works in his interviews; not the Expanded Universe.
Now some other contributors to Star Wars lore (who are possibly fanboys and fangirls of Mr. Lucas's works) have actually went out of their way to overhype some G-canon characters in the Expanded Universe such as Yoda and Sidious by labeling them as most powerful practitioners of the light and dark respectively. Among all of the authors of Star Wars lore, you will find Daniel Wallace favoring and hyping Sidious in most of his works.
This began with Matthew Stover who unnecessarily hyped Yoda in ROTS novel. Since Yoda was touted to be the strongest Jedi ever in this source, it was expected from other authors to take the liberty to hype Sidious in the same fashion in other works since ROTS movie depicted a fight between these two ending in a stalemate. This is where Mr. Wallace stepped in. However, this official "Yoda/Sidious wanking brigade" seems to have taken a break in recent times.
So what we have now is:-
ROTS novel (released in 2005): Declared Yoda as the most powerful Jedi ever.
SWTCE (released in 2008): Declared both Yoda and Sidious as most powerful practitioners of the light and dark respectively.
It shall be noted that James Luceno believes in superiority of Plagueis.
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Now among the authors of Star Wars mythos, some have fetish for ancient era lore and have come up with remarkably powerful creations of their own with the hopes of getting as much recognition as Yoda and Sidious have. This began with introduction of Marka Ragnos and Revan. Bioware (under leadership of Drew Karpyshyn) earned enormous fanbase (sort of like cult like following) with KoTOR and along with it came the belief in superiority of Revan. This dialed down (at least in debates in forums) with the release of sources such as ROTS novel and SWTCE. Obsidian (under leadership of Chris Avellone) also stepped in by strongly hyping up ancient era lore in all of its forms. KoTOR II not just hyped Revan but also introduced monsters of its own with Nihilus being the most prominent. Unfortunately, the sheer bad@ssry of Nihilus have been under-appreciated in canon.
Currently, Bioware (under leadership of Hall Hood) have stepped-up in its game and is doing a good job in promoting its most recent work SWTOR. It remains to be seen how these promotions will be received in future canon content. So far, Pablo Hidalgo (one of the authors of SWTCE) seems to have gone neutral in the light of recent canon developments; his latest work SWTERC (released in 2012) represents Star Wars history from real-world perspective and the author have not wanked/hyped Yoda and Sidious in it.
Finally comes the fan based interpretation of these matters:
The "Yoda/Sidious wanking brigade among fans" are adamant about the superiority of these characters over all others barring Luke. Ironically Luke have never been hyped in comparable fashion in canon as Yoda and Sidious have been but he "escapes criticism" by being part of the lore that goes beyond the existence of Yoda and Sidious (future). However, similar flexibility in opinion is not witnessed from the "Yoda/Sidious wanking brigade among fans" for predecessors of Yoda and Sidious, irrespective of their capabilities and accomplishments. Whenever this issue is brought up, the "Yoda/Sidious wanking brigade among fans" counters with an argument that all of the predecessors of Yoda and Sidious are supposed to be inferior to them because these two are the most powerful "in history" in canon. My personal counterargument in this case is that the sources which have hyped Yoda and Sidious (as most powerful) are now outdated in the light of latest canon developments and these two characters require re-evaluation accordingly. My point is that the accolades given to Yoda and Sidious in lets say SWTCE are valid in the context of the information contained in it.
To each his own but what I have noticed is that fanboys try to suppress one another in these matters which is a bad thing to do.
I believe that HoT can defeat Yoda (on the basis of his holistic picture) and I should not be blindly shunned by fans of Yoda for this claim. Similarly, I believe that Vitiate can defeat Sidious (on the basis of his holistic picture) and I should not be blindly shunned by fans of Sidious for this claim. I am not trying to devalue Yoda and Sidious but I am keeping an open mind.
I am not a blind believer in superiority of any character in the Star Wars mythos in-fact though I do have my favorites. Still I try to roll with latest developments. I do accept the fact that I find the ancient era lore of Star Wars most fascinating, but this is due to interesting stuff that have made it appealing to me. TOR is a nice alternative to G-canon works.
I consider myself a fan of both TOR and G-canon works. However, needless G-canon wanking have forced me to debate in favor of TOR era lore more often then I would want to.
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Legend:
ROTS = Revenge of the Sith
SWTCE = Star Wars: The Complete Encyclopedia
SWTERC = Star Wars: The Essential Reader's Companion
KoTOR = Knights of the Old Republic
SWTOR = Star Wars: The Old Republic
TOR = The Old Republic