Here are some mistakes I have seen in the thread, and I would like to clear them up before I throw my opinion of the winner out there. The quote when
Ancalagon was thrown down says he broke the
towers of Thangordrim, not the
mountains, so his size needs to be curtailed a bit. And
Luthien in her spell of growth(in the lays of
Beleriand) uses Glaurungs name, as being the biggest
dragon. It could be argued that Anc.. was not known or alive at the time, but she does use the name of the chain that was to bind
Melkor after the last battle,and so did not yet exist, I know from HoME that the final name of the chain was not set in stone, and it could have been the same chain that had bound him before. so the possibility exists she knew Glau.. was the biggest. The argument on Glaurungs spell ability is assuming Ancalagon did not have it. I do not think we can make that assumption, as
Smaug had some powers like Glaurungs, he almost got
Bilbo to fall under the "Dragon Spell". If Anc.. did have magic, then I would assume that they would each have been immune to the others magic. After all this, I must say I think Anc.. would win, he could fly, and so, if both
dragons were fire drakes, and could not breath on each other, he could at least drop huge boulders and other
objects on G, before closing for claw to claw. and as the more mobile dragon, he could fly around and choose his attack, while G would have to wait, and always be in a defensive position. The best reason he would win depends on if he was a fire
drake or not. In the Sil, when the winged dragons are released, they come with thunder and
lightning and fire. So there is evidence he was a different sort, and used more than fire.
So there you go, my opinion is Ancalagon would win 6 or 7 out of ten fights. I wait for responses to this so I can spend more time looking through my
books and avoid studying for finals