Dragons and Balrogs

[Son of Númenor]

Another reason I think Balrogs are stonger is because Tolkien wrote that the elves feared the Balrog of moria greater than anyone but he who sits in the Dark tower. (Sauron)

Well, there were no dragons for them to fear after Smaug died.

In the Silmarillion Tolkien says that other than Sauron, Gothmog was Melkor's strongest slave (for lack of a better word)

Can you provide a quote for this? It may be right, & I haven't got the Silm with me to verify or disprove, but I think I remember Glaurung being described as Morgoth's greatest servant besides Sauron.