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.
Going of what we knew before Tartaros, Igneel was talked about like he was the strongest among the Dragons by other Dragons, minus a few who considered Acnologia a dragon.
After Tartaros, we can assume that seeing Igneel was able to match Acnologia with only what little power he had left in a half-dead state, Igneel would probably be much stronger than Acnologia when fully alive and with his power back to him, but we know he was in a half-dead state because of Acnologia absorbing his soul while being a Dragon Slayer.
I am not sure if igneel did match acknologia... Surely he performed well but in the end acknologia seemed to have a somewhat overwhelming victory. If igneel was at any point much stronger than acknologia then I don't see how the events would have unfolded as they did. Why not just defeat him 400 years ago? Instead the result was that all the dragons today, including igneel, were already half killed by acknologia. If acknologia was able to kill all the dragons that were within the dragon slayers 400 years ago then to me it would seem like even now acknologia did not really show all of his power against igneel.
Igneel should be equal to Acnologia because Igneel was not at 100% when fight happened.
Igneel was shadow of former himself.
I'd say strong enough to just be slightly beneath acnologia at full power. That makes him superior to everything and everyone seen thus far.
I do not think ıgneel stronger than acno,I mean why didn't he killed him 400 years ago if he was stronger than him ?
I think he was almost same level with acno but not stronger.
And I do not want to get off the topic but why didn't other dragons fight against acno?If they attacked him at the same time then they probably take him down.
Acnologia minus an arm fled after landing the final blow to Igneel. I still think Igneel is stronger than Acnologia as he matched him when he had very little of his former power left. Acnologia couldn't of killed a Dragon as Powerful as Igneel 400 years ago, but could absorb his soul to weaken him so he may defeat him in the future. Thats what has seemingly happened.