Ancalagon and Glaurung should be the mightiest dragons (or even evil beings) of all time (Except maybe ungoliant).
I think Ancalagon was deadlier; he could fly which Glaurung could not, and in LOTR Gandalf states that a Dragon's fire could'n destroy the one ring, "Not even Ancalagon the black" This indicates that his fire is deadlier than Glaurungs.
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And with that he lifted high his sword and flames ran down the blade
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