[denethorthefirst]

If I remember correctly Tolkien stated that the power of some of the older Eldar (and especially if they had been re-housed, for example Glorfindel) was comparable to that of some of the Maiar. Galadriel was (in the Third Age) clearly on of the oldest remaining Elves in Middle-Earth and Tolkien even went so far in his later writings to state that she was the second most powerful of the Noldor (after Feanor): "Galadriel was the greatest of the Noldor, except Fëanor maybe, though she was wiser than he, and her wisdom increased with the long years" (from Unfinished Tales). Considering all that it seem plausible that Galadriel was at least as powerful as some of the weaker Maiar (for example Radagast, or one of the other hundred unnamed helper-spirits). From a story-perspective she has to be that powerful to be a credible opponent to Sauron (Tolkien mentioned that only Saurons personal, active presence could have worn down the defenses of Lorien). But it is also quite clear that she was still less powerful than Sauron and that she would not have been able to master the One Ring. Tolkien stated in a Letter (No. 246) that:

"Of the others [non-mortals in the story] only Gandalf might be expected to master him [the One Ring] – being an emissary of the Powers and a creature of the same order, an immortal spirit taking a visible physical form... [...] Confrontation of Sauron alone, unaided, self to self was not contemplated [by Galadriel]".

Sauron was spiritually more powerful than Galadriel. But while a battle between an Elda and an Umaia is in large parts a spiritual battle, the physical realities are a factor too - and the simple fact is that Sauron (in his unique incarnation) was also physically superior, a lot taller and bigger, heavier and stronger than the elf-woman Galadriel. In my opinion Sauron just outclassed Galadriel in every way: spiritually, physically, mentally. I think that in a hypothetical 1 on 1 confrontation Galadriel would not even stand a chance against Sauron, Ring or no Ring.

The One Ring seduced everyone in its reach and filled the minds with delusions of grandeur and power, like a euphoria inducing drug almost. Galadriel was obviously not immune to that effect, so she is not a reliable narrator regarding the One Ring and her ability to master it, another quote from the letter:

"In the 'Mirror of Galadriel', 1381, it appears that Galadriel conceived of herself as capable of wielding the Ring and supplanting the Dark Lord. If so, so also were the other guardians of the Three, especially Elrond. But this is another matter. It was part of the essential deceit of the Ring to fill minds with imaginations of supreme power."