system
Given the following post, respond as if you're commenting on Reddit:
human
[The Silmarillion] Which dragons under Morgoth's command was the most dangerous?
gpt
There are only two named Dragons that were under Morgoth's command. 

Glaurung - The first Dragon. Wingless. Smart, capable of using magic. Was one of Morgoth's commanders. Killed a Dwarf King. Destroyed Nargothrond. Played a major role in the Curse of Hurin. 

Ancalagon the Black - The largest and most powerful dragon to ever exist. He was so big that he crushed Thangorodrim when he died. 

So in terms of raw power, Ancalagon wins. But the most dangerous? Well, we don't really know much about Ancalagon. He sort of just shows up and dies. Glaurung is a much more fleshed-out character, with actual dialogue and personality. If we're talking about dangerous in terms of raw destruction, Ancalagon would win. But if we're talking about dangerous in terms of cunning, intelligence, deceit etc, Glaurung wins.