I would have to place all of the Maiar first on the list because, well, they were Maiar. In my view, they are therefore more powerful than Elves, even very powerful Elves such as Galadriel, Elrond and Glorfindel.
Sauron tops the list in my book, whether with or without the One Ring. Even without the Ring, he was the greatest threat to ME throughout the Third Age: he had massive armies at his disposal (not to mention assorted nasties like the Nazgul) and the major powers of "good" had a pretty tough time defeating him.
In my opinion, Gandalf and Saruman would have been a match for Sauron in their Maia form, but not in the human form which they took on in ME. While I am tempted to place Saruman above Gandalf, by reason of the forces that he evantually had at his disposal, I believe that Gandalf edges it by reason of his greater wisdom and humility (enabling him to avoid the corruption that spelt Saruman's doom) and also because he was able to mobilise the free people of ME to his cause. Whether Grey or White, therefore, I would put Gandalf before Saruman.
Than comes the Balrog, another Maia. Although Gandalf was able to defeat him, he died doing so, and I believe that he would have been a match for any Elf. Yes, Glorfindel had defeated a Balrog, and quite possibly the likes of Galadriel and Elrond could have done so, but I believe that the Balrog was intrinsically more powerful than them.
Then come the greatest of the Elves: Galadriel, Elrond, Glorfindel, Cirdan, Celeborn and Gil-Galad, in that order. Oh, and the Witch King would have to go after Galadriel, for the very good reason pointed out by Chancellen: no male could defeat him.
And I have to include Smaug. As one of the last surviving representatives of Ancalagon's horde, he was surely one of the most powerful beings of ME in the Third Age.
After that it becomes difficult. We have the likes of Thranduil, Dain II, Aragorn, Isildur, Elendil etc, not to mention the remaining Nazgul. It becomes very difficult to choose between them, so I won't. So, here's my list:
1. Sauron
2. Gandalf
3. Saruman
4. The Balrog
5. Galadriel
6. The Witch King
7. Elrond
8. Glorfindel
9. Cirdan
10. Celeborn
11. Gil-Galad
12. Smaug
Shelob doesn't make my list since, although fearsome, I could not put her above any of the 12 listed above. Aragorn would probably have been a pretty good match for her in a straight fight.
Also, I haven't included Tom Bombadil, since he cannot really be compared with the others, being the enigma that he is. Perhaps he should come first, being unaffected by the Ring, or perhaps he should not feature at all, being unconcerned in such matters.
There are others who might be included, but we don't know enough about them. The other Istari, for example, although I tend to agree with the comment on another thread that Radagast was a bit of a "rubbish wizard".

The two blue wizards? Any other Balrogs or dragons who escaped the War of Wrath? We just don't know enough about them.