Tolkien's best Dragon

[Orald]

Ancalagon is only mentioned in the War of Wrath, so the amount of lines concerning each dragon does not necessarily denote how great they are. The lines from War of Wrath read as thus:

Then, seeing that his hosts were overthrown and his power dispersed, Morgoth quailed, and he dared not to come forth himself. But he loosed upon his foes the last desperate assault that he had prepared, and out of the pits of Angband there issued the winged dragons, that had not before been seen; 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. Before the rising of the sun Eärendil slew Ancalagon the Black, 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.

Now that is a pretty big dragon that can crush the peaks of Thangorodrim. Plus it does say he was the mightiest of the dragon-host, and in the mightiest of the winged-dragons.