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.
Isildur used a broken sword to cut the Ruling Ring off Sauron's hand. Sauron was fatally wounded by Gil-galad and Elendil as Sauron killed them. Gandalf took the job of blocking the Balrog - and he was barely able to defeat the Balrog (in fact, he died in the process). A few reasons.
The war ended when Sauron himself came forth and fought Gil-galad in combat, who was then joined by Elendil. Although they both fell, they managed to defeat Sauron and bring him down. Isildur in the same hour then subsequently cut the One Ring from Sauron's hand, dispersing his spirit.
Sauron's quest for power dominates much of Middle-earth's history. It is his greed, however, that eventually leads to his ultimate destruction in The Lord of the Rings. By imbuing the One Ring with a part of his soul, Sauron sealed his fate by giving Middle-earth's heroes a way to destroy him once and for all.
Though wizards are technically Maiar, they do not possess all the same abilities in their wizard forms. This is why, despite Balrogs and Sauron also being Maiar, Gandalf wasn't evenly matched in his wizarding form—especially as Gandalf the Grey.
Gandalf stated that Sauron feared “something indomitable” within Gollum. By the exact language used, Gollum is indomitable, meaning he cannot be dominated. Sauron is all about domination - and Gollum was openly defiant - breaking under torture, but still openly displaying his hatred and wrath to Sauron.
However, I would consider that his Pride was his weakest point, his arrogance was a weakness of his character. Sauron was mighty and knowledgeable in the lore of the physical world, as he was a Maia of Aulë and he also learnt much from Melkor. However, his evil diminished his wisdom.
Finally Sauron came out to fight Elendil and Gil-galad face to face. When Elendil fell, his sword Narsil broke beneath him. Isildur took up the hilt-shard of Narsil and cut the One Ring from Sauron's hand, vanquishing Sauron.
During the First Age, Sauron was defeated by the combination of the efforts of Huan and the power of Lúthien, at Tol-in-Gaurhoth, and was forced to cede his power upon the fortress to the daughter of Melian and Thingol in exchange for his ability to flee.
In the books, Isildur did not defeat Sauron; it was his father Elendil and the Elf King Gil-galad who defeated Sauron together at the cost of their lives. Isildur cut the Ring from Sauron's dying body, and his spirit fled.
The story was abandoned, but Tolkien reused the characters and events. Isildur features briefly in voiced-over flashback sequences of Peter Jackson's The Lord of the Rings film trilogy. The video game Middle-earth: Shadow of War departs from Tolkien's narrative by having Sauron make Isildur into a Nazgûl or ringwraith.
Bloodlines are very important in Tolkien's stories, and it makes a lot of sense that Sauron would fear the great-grandson of the man who cut off his hand. Aragorn carried the blood of Numenor with him. His genetic connection to the heroes of the Second Age established him as a dangerous opponent of Sauron.
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.
Morgoth is the more powerful of the two. Morgoth was the mightiest of the Valar, a group whose weakest member would outpace Sauron. However, there are some circumstances and eras in which Sauron would win the day. Morgoth was born far beyond Sauron but lost and surrendered much of his power.
Since their will was not bound to Sauron now, it is more likely that they ran away in terror and scattered across Middle-earth. After Sauron's defeat, Aragorn and his army killed the remaining Orcs in Mordor.
Although Sauron's origins are angelic, he becomes captivated by the idea of ordering things as per his own will, which might be a possible reason behind him being lured by Morgoth, a Dark Lord who corrupted countless souls and waged wars against Elves and Men throughout the First Age.
Meanwhile, a few faithful Númenóreans, led by Elendil, were saved from the flood, and they founded Gondor and Arnor in Middle-earth. Sauron still considered them his hated enemies and he launched a pre-emptive attack on Gondor in S.A. 3429.
Mainly Eru for going against his will and siding with Morgoth. After the fall of Morgoth and Sauron's rise to power his primary nemesis would be a collection of leaders who opposed him: Gil-Galad, Elrond, Cirdan, Galadriel, Celeborn, the line of Elros and the Numenoreans.
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.
In his notes, Tolkien remarks that Sauron saw Galadriel as his equal. Add that to the fact that she's untrusting of Sauron (yes, even when he's in his Annatar guise) and an unwavering, single-minded force, and it's more likely than not that Sauron feared Galadriel — or the potential she held to upend his plans.
"So I think in his pitch to Galadriel, it cannot mean that he loves her or that there's any kind of romantic relationship. There should be no ambiguity around the fact that Sauron is evil — he's terrible, and he's using Galadriel to enhance his power."
While the book version of Sauron is Pure Evil, due to being a manipulative sadist, which was shown in The Silmarillion, his lack of personality and motives in both the Ralph Bakshi and Rankin-Bass movies renders him not even close to qualifying in those versions.
