People, love to throw around the letters of Tolkien, specifically number 246, when they argue against Galadriel being as powerful as she seem to be in these rather foolish movies (hate Jackson forever!!!). They don't consider several aspects of the ring lore.
For one, when Tolkien meditated on the use of the ring in this letter, he clearly talks about an individual who did not master the ring. That is if the ring holder would confront Sauron before they were able to master it, they would have probably lost (even Gandalf).
The other aspect would be that they don't understand what was the power of the ring, that went beyond the enhancement of personal power, like the other rings did. The one ring differed from all others because it had the key to hijack them, know everything the others did with their rings, including all military defence, location and so on. So if Sauron would have gotten his ring back, the realms of the Elves and the work of Gandalf would have been transparent to him, neither one could have done a move that could have surprised Sauron, even if they took of their rings. That is, the true power of the ring was not its enhancing effect, but its "internet" like nature.
If any one of the wise, who are immortal and cant be "shifted" to the other world, like mortals are, would have decided to take the ring, the first thing they would have done is learn how to master it, then they would have hijacked all the other rings, including the 3 dwarven rings as well as the nine that were used by Sauron as soon as he got them back, to enhance his power (as a "replacement" for the one). By doing so they would have seen clearly everything that was done with the help of those lesser rings, everything the nazgul ever did and everything Sauron did after his reincarnation. All his plans, and armies and strategies would lay bare before the ring user. So no move he could do could surprise the new ring master.
There is a secondary implication to this mastering. As soon as the ring would have been truly mastered, the power that was from Sauron in the ring would have been irrevocably separated from him, which would have resulted basically in the same as if the ring was destroyed, because for him it would have been destroyed. The new ring master then would have been able to take control of all his armies that he influenced through the other rings.
Another aspect that people ignore is the fact that out of all the rings the elves made Sauron most desired the three for they seem to have a power that is very distinctive from even the one, they seem to be able to influence the ageing of the world inside the aura of the ring which is large enough to cover a vast forest. Mastering one of the three must have been a great feat on its own. Whoever had thousands of years to master one of these powerful three would have had the chance to master the one as well.
An other interesting aspect is the fact that the nazgul were not afraid to cross to Imladris after Frodo, but they were afraid to challenge the power of Nenya.This is contrary to the general view that the three rings were not weapons of war. Clearly a device that has the power to slow down decay and the effect of time is incredibly potent. And a will that directs this power can and will use it to defend the region under its protection as Galadriel does.
Another aspect that many people forget is this, when Eorl tried to join Gondor in the war, Sauron from Dol Goldur sent out a black mist that almost engulfed his army, who knows what would have happened if it did. Luckily for him Gladriel countered this mist with a protective white mist that drove back the darkness. In this clearly magical confrontation Galadriel came on top. And yet another aspect is this, people forget that even without the one ring Galadriel had the upper hand as long as Sauron didn't had the one ring, for she was able to hack Saurons mind, while he could not see her, not even with the use of his palantir.
Many people also point to Finrod and how he lost a magical duel with Sauron, forgetting in their ignorance that the only reason Finrod lost that battle was because of the curse of the Valar, as it is clearly stated in the Silmarils. The Noldor were trapped between the curse of Mandos that acts as an oppressive fate and the will of Morgoth.This is why the wise Galadriel decided not to fight in the war against Morgoth, for in that constellation it would have been hopeless to win.
Also related to this question is the idea of some that Sauron with the one ring would have been invincible. When it is clear that Elendil and Gil-Galad manage to kill Sauron, while getting killed in the process, and only afterwards did Isildur claim the ring. Interesting this "act" of claiming the ring, it seem to be extremely powerful act. Even when small Frodo claimed the Ring for himself the power of Sauron was shaken literary. If a will strong enough to master the ring would have claimed it, the effect on Sauron would have been disastrous. No wonder he was so scared that his enemies found the ring, he attacked Gondor much more earlier. If there was no chance for any of the wise to master the ring, he would have had nothing to fear, for any one of them if they would have claimed the ring would have inevitably lost at the end. But apparently according to Sauron this was not so, him losing the ring to one of his enemies, especially the powerful elves, was a fear real enough.
It is also forgotten that the vision that Galadrien shows to Frodo when he offers her the ring, is created by the light of Earendil on Nenya, and the light that cam from Nenya. And as such I believe it to be true. For the three were unstained.
Some people also underestimate the skill of the elves, they believe that it was only Sauron who thought them how to make rings of power and how to use them. This is nonsense, both Sauron and the elves learned from each other, so much so that Tolkien states that Sauron used this collaboration to "learn all their secrets". We also know that the elves alone unaided were capable of creating a similarly powerful device, the Elesar, who in the hands of Galadriel made it possible for the Malorn trees to grow and the forest to be lovely. I don't know about you but the three rings seem to me only an enhanced version of the Elesar. And again, Sauron is stated to desire the three above all else, for this same reason.
All in all, I think the idea that Galadriel could have "vanquished" Sauron is not at all unrealistic, while neither had the one. And I also think that if she would have gotten the one, she would have been able to master it, and by doing so she would have destroyed Sauron without a fight.
And as a finish, two clarifications. One, there is no evidence that she created the phial that she gave Frodo until the day she gave it to Frodo. She created it especially for him. This creation in turn shows that she is a true Noldor, for her "engineering" skills are amazing. If she defeated Sauron at Dol Goldur she did it by her own spiritual power augmented by Nenya. Two, one of the reason that Sauron seems so powerful is the fact that he was able to use the Palatir as a channel for his will. We can see this when he is searching for Frodo while Frodo is using the ring on the mountain top. Where the will of Sauron collides with the will of Gandal, in Frodos head. We can also see the same when Frodo sees the window of the palantir for a moment in Mordor and he feels like is was struck. We can all imagine how augmented by the nine and the dwarfish rings using the palatir as a channel he was able to direct his armies so successfully. Of course all of those war plans and armies were prepared by him and his nazgul in centuries past with the use of some of the rings. So my advice is this, stop thinking like a gamer geek, of elves and maias and how one must be always more powerful, when this is clearly not the case. Try to think in a more sophisticated and nuanced way about this issue. All feas are different aspects of Iluvatar, and they are not static, they can increase and decrease in power and potency.