The most evil being in The Lord of the Rings is generally considered to be Morgoth (Melkor), the original Dark Lord and source of all evil in Middle-earth, though Sauron, his chief lieutenant, serves as the primary antagonist in the Lord of the Rings narrative and embodies evil through control, corruption, and domination, representing a profound, pervasive malice.
Across three parts, the novel describes the Third Age rise to power of its titular tyrant, Sauron, the Lord of the Rings. Literature's greatest villain and by far the most well-known in The Lord of the Rings, Sauron was actually following in the footsteps of an evil that may have been greater than him.
while Sauron was more powerful than Gandalf in a direct comparison, their roles and methods were fundamentally different. Gandalf's strength was in enabling others to resist evil, while Sauron sought to dominate and enslave.
While Sauron embodies pure evil, seeking dominion over all, Saruman starts as a figure of wisdom who ultimately falls prey to his own ambitions. The key differentiating factor is that Saruman, in his quest for power, attempts to rival Sauron but ends up becoming a pawn in his game.
Gandalf as Ring-Lord would have been far worse than Sauron. He would have remained 'righteous', but self-righteous. Thus while Sauron multiplied evil, he left 'good' clearly distinguishable from it. Gandalf would have made good detestable and seem evil.
At the end of the second age, the High King of the Elves, Gil-galad and Elendil, the King of the Numenorians in Gondor, defeated Sauron in armed combat,  although they both died in the fight. There is a long history of Elves or extremely powerful men defeating Miar - like the Balrogs.
His task was to do the bidding of his masters and so when he proved unfaithful in doing that task his masters stripped him of his robes and gave them to one who would do their bidding. Gandalf is now the White, Saruman as he should have been.
Sauron was originally a Maia of Aulë the Smith; his name then was Mairon, meaning "the admirable." He learned much from Aulë in the ways of forging and crafting, becoming a great craftsman, and mighty in the lore of Aulë's people.
Is the Necromancer actually Sauron? Yes, the Necromancer in Dol Guldur is Sauron. He disguised himself as the Necromancer so that he might build up his forces and power and once again try to overtake Middle Earth. He needed to hide his identity from the Elves as long as possible to gather his power.
In Valinor, Gandalf, a Maia, was named Olórin, one of the people of the Vala Manwë, and the wisest of the Maiar.
Sauron was the most powerful of the Maiar—primordial spirits created to help the Valar first shape the World. His original name was Mairon, which means "the Admirable," and as an immortal spirit born before the world's creation, he was able to perceive the Creator Eru Ilúvatar directly.
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The hobbits, followed by Gollum, reached Mount Doom, where Frodo was overcome by the Ring's power and claimed it for himself. At that moment, Gollum bit off his finger, taking back the Ring, but, gloating and dancing incautiously over the edge of the precipice, he and the Ring fell into the fires of Mount Doom.
Why was Sauron afraid of Aragorn potentially possessing the Ring? Gandalf explains it in the Return of the King (during the council after the battle of the Pelennor fields) that Sauron would never think somebody would want to destroy the One Ring.
As one of the Maiar, Gandalf was not a mortal Man but an angelic being who had taken human form... along with the other Maiar who entered into the world as the five Wizards, he took on the specific form of an aged old man as a sign of his humility...
Gandalf does not WANT to touch the ring. Just as Galadriel resists the temptation to take it when Frodo offers it to her willingly. And all this because "Power Corrupts". Sauron and the Ring are allegories for the corruption that inevitably arises when too much power is concentrated in one place.
'The Fellowship of the Ring' (2001)
If Gandalf had stayed dead after his sacrifice in The Fellowship of the Ring, then his demise would be the saddest in the trilogy, but since he came back, the saddest single death scene of all three movies goes to Boromir's.
Well, there's also a hierarchy of power within the Maiar as well -- Saruman is more powerful than Gandalf, by his own admission, and Sauron is maybe the strongest of all the Maiar, being the chief lieutenant of Morgoth himself.
Gandalf's quest in middle earth was not to fight. His presence was merely meant to help the people's of middle earth rally to defeat Sauron with their own strength. Gandalf was there to help with some magic ability to combat so of the magic of Sauron, but he was not sent to middle earth to be a warrior.
The most powerful were probably Eönwë, Ilmarë, Sauron (Mairon), Melian, Arien, Ossë, and Uinen. The writings do say that Gandalf (Olórin) was reluctant to be sent as one of the Istari because he feared Sauron.
Gandalf is not permitted to use his full power during the 3rd Age (some exception apply). He has been sent to guide humans to the 4th age, the age of mortals. He can perform minor magic as needed, but he cannot "bust out."