Nenya, as the other elven rings was not an instrument of combat or power, it was intended to help heal and preserve things and would slow down the fading of the
elves in
Middle Earth.
I don't believe any
elf, even
Feanor or
Fingolfin were even close to the
Maiar.
Sauron's power was diminished and still, it was needed the combined power of one of the most powerful elves (
Gil-galad) and one of the most powerful
men (
Elendil) to defeat him, yet, even though
Isildur cut
the ring off the hand of Sauron, he wasn't completely defeated.
As per
Galadriel, she was a high-elf, not many remained in Middle Earth in
the third age. She was very old, had learned a lot from
Melian and she was a descendant of a house of
Kings. Daughter of
Finarfin, granddaughter of
Finwe, no wonder she was powerful but more powerful than the Maiar? I don't think so.
Also, the powers and abilities of elves and maiar are not all the same, some excel at one area, some at others.
Galadriel had a lot of wisdom, she could communicate with her mind (but so could
Celeborn and
Elrond). She could read Sauron's mind while shielding hers from him. In that sense she was incredibly powerful and I guess a lot of it came from her time with Melian.
But
Glorfindel was more powerful in battle, and Elrond in healing.
Among all elves no-one was more skilled in mind and body than Feanor, nobody was stronger in battle than Fingolfin.
Finrod, Galadriel's brother also had amazing magical powers as well as combat strength (he killed a werewolf with his bare hands).
So, no doubts that because of age and genealogy Galadriel was likely the most powerful elf left in Middle Earth, but she wasn't more powerful than many elves of the
first age.
I don't think there is a clear match-up between elves and Maiar. Sauron against Gil-Galad but Sauron wasn't at the top of his power. Melian clearly outclassed anybody else in
Beleriand.
Eonwe was the best warrior the
Valar could send (since
Tulkas was kept away to not make to much of a mess) Arien was feared even by
Morgoth.
The
Istari don't count as they were weakened.
Osse and Uinen demonstrated their powers, way beyond what any elf could do.
Others like
Ilmare, or Tilion weren't ever involve in any kind of fight.