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.
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…
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.
From a technical sense, Sauron and the Balrogs were Maiar, not Valar. That meant that they were designed as lesser beings from the beginning of time. Thus, in their original forms, Morgoth would have easily defeated Sauron in a fight.
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.
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.
It was said that Morgoth hated and feared the House of Fingolfin most among the Houses of the sons of Finwë, especially Fingolfin's son Turgon, as it was prophesied that his doom would come from the House of Turgon.
She had no role in the major wars of the First Age, believing that defeating Morgoth was beyond the power of the Eldar; when Morgoth was defeated by the coming of the Valar out of the West she, having played no part in the earlier Kinslaying, was offered safe return to Valinor.
"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."
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.
Morgoth, the first to reign terror upon Middle-earth, was more powerful than Sauron and commanded an army of dragons and Balrogs. While both Morgoth and Sauron were cunning, Morgoth's rule and terror lasted longer and his corruption spread further, making him more formidable in the Lord of the Rings universe.
As they did when the elves were first born, the Valar came to Middle-earth and defeated their brother, but this time Morgoth was cast into a void outside of existence, where he remains thereafter.
Sauron hated the way Arda was run. He was the enemy of the Valar, because he thought they did a poor job organizing it. Morgoth was the GREATEST enemy of the Valar, and the most awesome being in the universe, and thus he became Sauron's boss.
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.
Morgoth clearly does have a motive, having a great deal of pride, desiring the power of creation that Eru Ilúvatar possessed, and becoming bitter for being the only one with it. He also wished to have power over Arda and began to destroy it when he was refused this.
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.
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.
For those who have read the Silmarillion, it seems that it were the Valar who defeated Morgoth. Whatever role the Valar played in the War of Wrath, it did not involve any personal confrontation with Morgoth in Angband, for was Ëonwë who personally did the job.
Thus, Galadriel's dark form in Jackson's The Lord of the Rings personified what would have occurred had the Lady of Lothlorien given in to temptation and taken the One Ring for herself. She would have succumbed to evil because the Ring was indomitable, even with her power.
She also resisted being offered the ring of power by Frodo following his vision in the mirror of Galadriel, an immense test given the ring's seductive powers. Yet Gandalf's power level exceeds even that of Galadriel, as shown by his rebirth in the Two Towers.
The outcome is that Galadriel is able to penetrate Sauron's mind and issues a single command "Go back to the void from whence you came!" Unable to resist her any longer, Sauron screams and flees with the Nine.
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.
The Balrog, like Gandalf, is a Maia. There is a very real possibility that Gandalf will die in this confrontation. As an istar Gandalf is an incarnated maia, and thus subject to all the frailties and fears that Elves and Men are subject to.
Gandalf was a Maiar, he already knew the Balrog since before he became an Istari in Middle Earth and named Gandalf. Being the Maia Olorin, the Maia of Manwe he already knew about Morgoth and his servants since he participated in the War against Morgoth and his servants.
