The answer is, by far, Melkor. Melkor, or Morgoth, wanted to currupt and destroy everything good created by the other Valar. Sauron on the other hand only wanted to control and rule Middle Earth. The rings were a way to control men, elves and dwarves, not to destroy them.
Morgoth Was Originally More Powerful Than Sauron
When he couldn't accomplish that, he began to war against the other Valar and rule over Middle-earth with his fires and evil creations. With this turn to darkness, Melkor became known as Morgoth.
Sauron: The Big Boss At The End Of The Game
Do you realize that The Lord of the Rings gets its title, not from its protagonist, but from its main villain? From his dominance in the Second Age, all the way to the terror that he wielded during the trilogy, Sauron was peerless in his streak of evil.
Melkor is the most powerful of the Valar but he turns to darkness and is renamed Morgoth, the primary antagonist of Arda. All evil in the world of Middle-earth ultimately stems from him. One of the Maiar of Aulë betrays his kind and becomes Morgoth's principal lieutenant and successor, Sauron.
The Dark Lord Sauron is the most powerful villain in Middle-earth. He is a Maia, one of the divine spirits who entered the world to aid the Valar in their work. Sauron is responsible for creating the One Ring, which grants him immense power and the ability to control others.
Admittedly, the Witch King takes some power from Sauron, but of necessity it is less than Sauron himself possesses (which may or may not be enough in its entirety to destroy Gandalf the White, but definitely is not when only a fraction is brought to bear).
Characters who can easily defeat Sauron are; his Creator Eru Ilúvatar, his old master Melkor, and the Valar. Others who could overcome him would be Ungoliant who physically dwarfed Morgoth, and some great Maiar of Valinor described as almost as great in power as the Valar themselves.
For two and a half thousand years, Sauron ruled Mordor uninterruptedly. It was from Mordor that he made war against the Elves of Eregion and came to dominate most of Eriador in a period known as the Dark Years.
God is the most powerful entity in Tolkien's Lord of the Rings universe. The Elvish name for him is actually Eru Ilúvatar, meaning “the one, father of all.” So the question becomes: Who is the second-most powerful being? Originally, it was Melkor, “he who arises in might,” the most powerful of the Ainur (or angels).
Eru is introduced in The Silmarillion as the supreme being of the universe, creator of all existence, including the world, Arda, and its central continent, Middle-earth. In Tolkien's invented Elvish language Quenya, Eru means "The One", or "He that is Alone" and Ilúvatar signifies "Allfather".
It begins after Sauron's master, the evil lord Morgoth, is defeated. Sauron may have gone into hiding, but he's still alive: the series will eventually show Sauron's creation of the titular rings of power—including the one ring to rule them all.
Eru Ilúvatar, also known as the One, is the single omniscient, omnipotent, and omnibenevolent creator. He has been existing eternally in the Timeless Halls and possesses the Flame Imperishable in his spirit which kindles existence from nothingness.
Morgoth's rule and terror over Middle-earth allowed Sauron to rise to power and continue his legacy, but there were only a handful of beings more powerful than Morgoth in the entire history of Middle-earth. Sauron, even at his most powerful, would be no match for his old master.
The Elven King named Gil-galad fought side-by-side with Elendil, the King of Men. Together they managed to mortally wound Sauron, but the fight cost them their lives. This is when Isildur, son of Elendil, took his father's sword and cut the Ring from Sauron's hand, dealing the final blow.
Different: Sauron Is Probably More Powerful Than Voldemort
Voldemort might have descended into the depths of darkness, but Sauron is an entity from another time, possibly with powers that even Voldemort cannot imagine.
Weakness: The One Ring
The One Ring is Sauron's greatest power, and, at the same time, his own poison. It gives him the strength he needs to dominate Middle-Earth, but it also holds the secret to his own destruction. Even his immortality can be taken away once the ring is destroyed.
A blue whale can generate a maximum force of approximately 60 kilonewtons. Whales can claim the title of the strongest creature on the planet simply due to their enormous size. Although unable to lift, grip, or kick, they do need an enormous amount of force to power their massive bodies through the water.
He will fight in the Last Battle against the Valar and their allies, but will ultimately be slain by Túrin Turambar, the Man he cursed. By finally defeating Morgoth, Túrin will avenge not only himself, but all members of the race of Men.
Sauron nor Morgoth are the most powerful in Tolkien's world
In Tolkien's lore, the most powerful being is Eru Ilúvatar, which means "the one, father of all” in Elvish. The author purposefully created him to reflect his Judeo-Christian beliefs. Eru is God, while Morgoth is the Devil, according to Biblical references.
Saruman, also called Saruman the White, is a fictional character of J. R. R. Tolkien's fantasy novel The Lord of the Rings.
Théoden is introduced in The Two Towers, the second volume of The Lord of the Rings, as King of Rohan. By this point Théoden had grown weak with age, and was largely controlled by his chief advisor Gríma Wormtongue, who was secretly in the employ of the corrupt wizard Saruman.
After the defeat of Sauron in the War of the Last Alliance, Mordor was taken under the control of Gondor, and the Gondorians built fortifications around it to prevent the return of evil things.
Sauron's deepest fear was that Aragorn would use the Ring against him. With its powers of domination, Aragorn could comfortably take over the entirety of Sauron's dark army. Though this would eventually corrupt Aragorn, it would allow him to destroy Sauron and take his place.
Would Sauron be able to taint and subsequently command Elrond, Galadriel and Gandalf through the use of their own elven rings, like he does with the Ringwraiths? In theory, yes. It would be possible for him to control them using the ultimate ring of power, in the same way that an alpha wolf controls its pack.
Sauron feared her
Among Tolkien's commentaries and notes, however, is the statement that Sauron himself saw her as his equal, and was consequently worried about what she might (and could) do if she so chose.