Just one thing I'd like to see mentioned, Ancalagon is not explicitly named the mightiest dragon, but the mightiest winged dragon. I had a rather long discussion in another forum with a fellow member on this topic. he considered that Glaurung was better.
I too believe Ancalagon is more powerful, not only because he is said to have a stronger fire-breath and also the ability to fly, but also as a latter dragon must have had certain "improvements" from the original design.
His main argument was that if he really was the mightiest of all, Tolkien would not have clearly specified mightiest of the winged dragons.
Since the matter is somewhat unclear, I'd be careful about this.
