Ancalagon (aka Acalagon the Black) is a dragon and my favourite character because he was the greatest of all winged dragons and the largest dragon to exist in Middle-Earth. He was bred by Morgorth and was unleashed as the "last line of defense" when Morgorth was facing his final defeat in a battle against The Host of Valinor during the First Age and they were driven back from the gates of Angband.
I like him because of how he was portrayed in the story; big (size unknown but said to be as big as a mountain), black, had flames that were hotter than any dragon, and was used as the ultimate weapon to push back The Host of Valinor.
Later in the Fellowship of the Ring, when Gendalf told Frodo about the origins of the One Ring, he mentioned of Ancalagon saying, "It has been said that dragon-fire could melt and consume the Rings of Power, but there is not now any dragon left on earth in which the old fire is hot enough; nor was there ever any dragon, not even Ancalagon the Black, who could have harmed the One Ring, the Ruling Ring, for that was made by Sauron himself."
So not even the greatest dragon can put a scratch on the One Ring which shows just how powerful the One Ring can be.
I loved his origin so much I even named my PC on PCPartPicker (my link doesn't work anymore, banned on that site) after him. My PC is black and red as well.