Though Glaurung was named Father of Dragons, Ancalagon, also known as Ancalagon the Black, was one of the most powerful and infamous of all the dragons bred by Morgoth during the first First Age and possibly the largest dragon to have ever lived. And his fire was thus far hotter than most other dragons.
Ancalagon's size is not specified, but may be gigantic due to the destruction of "the towers of Thangorodrim", which are elsewhere identified with the three smoking peaks of the mountain. He was however, said to be as big as a mountain. However, his size is never directly specified and some powerful but small creatures are able to cause great destruction even in death, such as Durin's Bane breaking the slopes of Celebdil. Like all other Urulóki, Ancalagon breathed fire, which was said to be hotter than any other dragon's flame.
Powers and Stats
Tier: At least 7-A, likely 6-B
Name: Ancalagon the Black
Origin: Lord of the Rings
Classification: Original winged dragon
Attack Potency: At least Mountain level (Destroyed the towers of Thangorodrim just by falling), likely Country level (One of the strongest servants of Morgoth. Dragons where the only creatures stronger than Balrogs, and Ancalagon was the strongest dragon)
Lifting Strength: At least Class E, possibly higher via sheer size (The size of a small country)
Striking Strength: At least Mountain Class, likely Country Class
Durability: At least Mountain level, likely Country level
Range: Hundreds of kilometers due to his size alone (So large he can blot out the sun even from very high up. The size of a small country)
Standard Equipment: None notable
Weaknesses: None notable