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Igneel is the King of Fire Dragons.
I'm pretty sure he is the strongest dragon to exist if you don't count in Acnologia as that one is a human that became a dragon.
Maybe each dragon has divided up into a category based on elements, in which they have their strongest member. Igneel is the strongest fire dragon, Gajeel and Wendys might be the strongest in their element and so on. I hope Laxus meets a lightning dragon so he can learn from the original himself.
Acknologia bathing in the blood of hundreds of dragons probably gained power beyond even them, which makes sense.