I say
Glaurung, look at all the destruction he caused and nobody was able to defeat him for a long time.
Ancalagon was killed in the first battle he showed up in.
The "first battle" Ancalagon participated in, was also the greatest battle there ever was - they were up against the host of the
Valar for crying out loud. Plus it was Ancalagon & co. that drove back the forces of the Valar, until
Eärendil showed up and slew him. Ancalagon's fall caused the
mountains of
Thangorodrim to crumble to bits.
xD ... So True..
Let's examine some evidence..
Glaurung:
-Ravaged ard-galen, driven back by
Fingon (
elves)
-Dagor Bragollach:
Was teamed with not only an entire army
He also had an enormous army of
orcs, and all the
Balrogs
And by the way,
Morgoth unleashed the fires from the Iron mountains...
-Nirnaeth Arnoediad:
Had a brood of lesser
dragons and undoubtedly an enormous host of orcs
-Sack of
Nargothrond:
Faced
Men and Elves, with another host of orcs
-Killed by a man (
Turin) on the ground
-Had
dragon enchanting spells/Dragon eye/
evil intelligence
Ancalagon:
-"was the largest dragon to have ever existed in
Middle-earth."
-Ancalagon and his armada of winged fire-drakes by themselves
drove back the Host of the Valar during the
War of Wrath at the end of the
First Age.. Thats the Entire host of the
armies of
Valinor, Valar,
Maiar, all arrayed in battle....
-"and so sudden and ruinous was the
onset of that dreadful fleet that the host of the Valar was
driven back, for the coming of the dragons was with great
thunder, and lightning, and a tempest of fire"
-
"But Eärendil came, shining with white flame, and about
Vingilot were gathered all the great birds of heaven and
Thorondor was their captain, and there was battle in the air
all the day and through a dark night of doubt"
So
Earendil, (An ascended man) who travelled upon the vessel Vingilot in the sky (Hallowed by Valar), with ALL the great birds of heaven- including Thorondor- the Greatest Eagle of all time? It took these guys all day and through a dark night of doubt to kill the uruloki fleet..
-Then when he died, he literally smote Thangorodrim beneath him. He literally broke the 3 peaks of the largest mountains in Middle Earth.
-"the mightiest of the dragon-host, and cast him from the sky;
and he fell upon the towers of Thangorodrim, and they were
broken in his ruin"
-"nor was there ever any dragon, not even Ancalagon the Black, who could have harmed the One Ring"
Of all the dragons
Gandalf chose to reference.. he chooses Ancalagon.
The conclusion is rather obvious... Ancalagon the black was the greatest of the winged dragons. Ancalagon smote Thangorodrim in his wake. It took an army of enchanted cisrcumstances and the greatest flock of
Eagles Arda has ever seen to kill him. Ancalagon not only would have dwarfed Glaurung in stature, but would have made quick work of him on the battlefield.
Ancalagon wins. EZ.