Like Melkor, all of the following happened before LOTR.
Both Tulkas and Oromë can better Melkor in direct physical combat. Melkor also feared and hated Varda the most despite being more powerful.
Ungoliant in a bloated state, also overwhelmed and would have consumed Melkor if it was not for his Balrogs coming to his rescue.
Sauron was a craftsman rather then a warrior, so there would have been quite a few Maiar who could defeat him in direct combat. He was defeated in physical combat by Huan the hound (who admittedly was protected by a prophecy). The most powerful character he defeated was Finrod Felagund (although he was under the curse of Fëanor)
Sauron later used his skills and knowledge to forge the one ring which greatly increased his power. He would likely be more powerful then any other Maiar.... but wither he can beat them in a direct fight is still no certainty.
Sauron at his peak, while wearing the Ring was defeated by Gil-galad and Elendil (although both also died). Sauron by the time of LOTR was much weaker.
No-one knows, not even Tolkien himself knew. There are some wierd theories out there though. Just google "who was tom bombadil" and look at some of them. The freakiest is some woman who says he is the reader. Some people have too much time on their hands.
Character I think had the greatest achievement has to be.............................................................................................................Bilbo Baggins for being able to do something no-one else could and that is give up the One Ring willingly of his own free will.