What did Sauron do to become Melkor/Morgoth's chief lieutenant?

Question

Morgoth, the creator of all bad things in the Tolkien universe, picked Sauron of all things to be his most trusted officer. Why was this? I have been reading the the pre-LOTR stories, and there seem to be much better candidates to choose from, like:

1. Gothmog, Lord of the Balrogs

2. Glaurung, Father of Dragons

3. Ungoliant, a very powerful dark spirit

These three have all done great and terrible things to Arda and its inhabitants on greater scales than Sauron, so why would Melkor/Morgoth choose Sauron, who did nothing (in my opinion) worthy of recognition?

Answer 1

The best explanation we have is given in the essay on Orcs (of all places) in Morgoth's Ring:

Though of immensely smaller native power than his Master, [Sauron] remained less corrupt, cooler and more capable of calculation. At least in the Elder Days, and before he was bereft of his lord and fell into the folly of imitating him, and endeavouring to become himself supreme Lord of Middle-earth. While Morgoth still stood, Sauron did not seek his own supremacy, but worked and schemed for another, desiring the triumph of Melkor, whom in the beginning he had adored. He thus was often able to achieve things, first conceived by Melkor, which his master did not or could not complete in the furious haste of his malice.

^{History of Middle-earth X Morgoth's Ring Part 5: "Myths Transformed" Orcs}

The short version is that Sauron was cunning, and got results, and those both count for a lot. What he got results in, however, is less clear; this particular essay discusses the creation of Orcs (one version, anyway), but in general we know precious little of what the Forces of Darkness were doing in the early Elder Days.

It's worth noting that Sauron did accomplish his share of things during the War:

• He took Tol Sirion and the watchtower of Minas Tirith
• He hunted and killed Barahir, father of Beren and thorn in Morgoth's side
• He killed Finrod Felagund, a great king of the Eldar

His one major defeat (that we know of) was when Lúthien destroyed his tower on Tol-in-Gaurhoth; but Morgoth can't really give him a hard time for that, considering how well he handled Beren and Lúthien.

Answer 2

Sauron was a better, safer and smarter choice compared to Ungoliant and Glaurung.

Melkor was both fearful of and unable to control Ungoliant. She had her own will and desires. She was both a threat and formidable foe for him. In fact had it not been for the aid of a number of Balrogs, who managed to drive her away, it's possible she may have devoured him. Anyway her hunger got the better of her and she devoured herself (how one manages to eat one's own mouth is beyond me, but I digress).

Glaurung whilst a devastating foe against the Elves, was much lesser than that of Gothmog or Ungoliant, not a Maia or something from the Unlight, merely an experiment of Melkor's. Once he sacked Nargothrond and had collected all the treasures of Felagund he spent much time simply guarding his hoard; thus this being of more importance than being helpful or serving Melkor.

Gothmog may have been the only real good choice as a Chief Lieutenant instead of Sauron. As Lord of the Balrogs, he had many impressive victories under his fiery belt. He was the only real equal to Sauron, appearing first as Captain and later as General of the armies of Angband. In fact had he not been destroyed by Ecthelion he may have proved to be the better choice as Sauron was ultimately quite disloyal and cowardly (this would have been unknown to Melkor at the time).

Melkor would have found Sauron's cunning and ability to convey information through spying very useful. Sauron, like Gothmog had a terrible, powerful physical presence, but Sauron also had powerful magical abilities as a sorceror. I think this would have tipped the job in Sauron's favour. Plus for a variety of reasons he was the only one against those that you mentioned that actually survived long enough to be of continued worth.