Sauron was never described as eviler than Melkor, nor obviously a greater threat, for the difference in stature and power between them was insanely vast, and of malice still existed. Sauron was a minor spirit, a mighty (indeed of the greatest bar very, very few) one but still a lesser spirit, Melkor…
However, the mere fact that the Valar didn't prioritize perusing Sauron proved he was a much lesser threat than Morgoth. It made perfect sense because Morgoth was a Valar, and Sauron was only a Maia. So, while Sauron was evil, Morgoth was clearly much worse.
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.
In terms of who accomplished more the answer is Morgoth. He's the prime antagonist in all of Tolkien's work. From warring with his brethren, corrupting Sauron, creating orcs and dragons, corrupting and destroying the Noldor, actually shaping Middle Earth, corrupting the very fabric of reality with his essence…
In The Silmarillion, Tolkien tells us about this brief period, in which Sauron was regretful of his servitude precisely for knowing the fate of Morgoth and wanting to avoid it for himself.
Why Didn't Sauron Use the Balrog? Middle-Earth Lore
Why didn t Sauron free Morgoth?
Well basically Sauron was terrified of the Valar and knew that it wasn't even remotely possible to free Morgoth from them. The Valar were far above him in every way as he was only Maiar, and his only way of standing against them was when Morgoth was at the helm, which still wasn't enough.
It is said in *Of The Rings of Power and the Third Age* that Sauron did the bare minimum to avoid punishment after the War of Wrath: “When Thangorodrim was broken and Morgoth overthrown, Sauron put on his fair hue again and did obeisance to Eonwe, the herald of Manwe, and abjured all his evil deeds.
The Valar really should have gone after him, but they were tired from the War of Wrath, and they had already drowned all of Beleriand. They didn't want to create any more destruction. This is also how some of the dragons and at least one Balrog survived.
In this case, Morgoth was more cunning, as his rule and terror lasted for an unfathomable amount of time, and his corruption spread further than Sauron could ever muster. It could be argued, however, that in an emotional and perhaps political sense, Sauron was once superior to Morgoth.
Additionally, while he was in his weakened state, Galadriel could hold him off through the power of will alone. However, if Sauron had his One Ring and faced off in a battle against Galadriel, it's unlikely that there would be much she could do.
Gandalf's main reason for supporting Thorin's quest in The Hobbit was to eliminate the threat of the dragon Smaug, who would be a powerful ally for Sauron. Despite popular belief, the Balrog known as Durin's Bane was more powerful than Smaug in Middle-earth, being a literal, primordial demon.
Originally known as Melkor, Morgoth is essentially the Lucifer of Tolkien's story. Like the devil, Morgoth was one of the first children created by the God of Tolkien's world, but succumbed to jealousy and greed, especially over the creation of other races, and was duly disowned by his fellow Valar.
He is described as being equal in power to Manwë, chief of the Valar in Arda. But his power increases in later revisions of the story until he becomes the most powerful among them, and in a late essay more powerful than all of the other Valar combined.
Even the most powerful Maiar, such as Melian and Gandalf, acknowledged the superior power of the Valar. Therefore, it seems unlikely that Sauron would be able to defeat any of the Valar in combat.
However, near the end, Morgoth had dispersed so much of his own power into the world, that (Tolkien wrote in a private letter) he was actually weaker than Sauron. Sauron made torture a specialty. Morgoth tortured people too, but Sauron was really into it.
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.
Ultimately, the only being more powerful than Morgoth in his prime was Eru Ilúvatar. Only the combined effort of the Valar could stop Morgoth, and only after countless battles. Sauron might have bested his mentor under certain circumstances, but Morgoth is the greater Dark Lord by every metric.
Eventually, Morgoth was bound in chains by the Valar and thrown into the Void, leaving the permanent damage his evils had done, and his former lieutenant Sauron, to trouble the world. One day, according to a prophecy, Morgoth will rise again in great wrath, but he will be destroyed in the Dagor Dagorath.
The dwarves of Middle-Earth were notoriously strong willed, able to resist corruption and stay true to their purpose. Sauron had no means by which to corrupt them regardless of how he tried.
Because of this they're essentially considered a lesser species in Middle-Earth, according to King Durin III in Rings of Power. This animosity and prejudice might be why the dwarves didn't feel the need to aid the men and elves during The Lord of the Rings, even though they were also fighting Sauron.
The Valar(and the Maiar) did not have authority to harm the Children of Ilúvatar. Therefore when Ar-Pharazôn the Golden arrived at the shores of Aman, leading the vast army of Númenor, they didn't know how to deal with the situation, and called out to Eru.
Sauron seemed to genuinely feel a connection to her, and part of that came from sensing the darkness inside of Galadriel. In that regard, his offer was really a seduction to the dark side.
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.
After Morgoth's ousting by the Valar, Sauron became the second Dark Lord and sought to conquer most of Arda through guileful trickery, with the Rings of Power or with overwhelming armies.
