Ancalagon the Black, the absolutely gargantuan dragon from the first age of middle earth!

This fantastic artwork is by Ruben de Vela. I think the sense of scale and immense power are remarkable.

But Earendil 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 Earendil 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. The Simarilian chapter 24