I will be putting the dragons in front of Zeref with Fairy Heart because we don’t know whether it would have worked against them or failed like all other magics but Dragon Slayer magics have. They said it might work but never said it was a sure thing.
Hands down. He is as strong as everybody else combined +9999. He fought with Igneel through the ground like it was air. That is the kind of feat DC Comics gives to Kryptonians to show strength. Don’t forget that Acnologia had a ginormous body and wings dragging through the ground making it even worse. They only show him destroying an island but the way he easily outclassed a casual continent buster tells us he is way beyond that level. His ability to replicate any magic or even other nonmagic materials he eats almost assures that his variety of powers is endless. Also, he is 400++ years old.
2. Irene:
She was the casual continent buster that could hold her own vs Acnologia even if it was just a minute. She could literally change the shape of a continent to the precision of a single person without accidentally killing anyone(that we know of) on it. That is the strongest feat in the entire series and she did it in her WEAKEST FORM. If Acnologia wasn’t around there wouldn’t be anything anyone could do to stop her from killing off humanity by herself one continent at a time. To put it in perspective, even Erza and Wendy’s combined plot armors weren’t enough to defeat her. She had to kill herself which was unsatisfying but there were not many other options. Also, she is 400++ years old.
3. Natsu:
He is not in the same league as the previous two. He is without a doubt at nation busting level but so are pretty much all the Spriggan 12. What makes him special is his ability to burn just about anything including space-time, souls and magic itself(signaled the end of antimagic butt kickings and, saddeningly, pocket-sized Natsu jokes). This power was an auto-win vs non-dragonslayers which was the only reason he could beat Zeref who was literally infinitely powerful at the time of their final battle. Unless someone can eat magic, they can’t tank this guy. Fortunately, that included a paralyzed Acnologia. Also, he is [technically] 400 years old with the mind of a 12-year-old(it will probably take another 400 years to fix that).
4. Zeref:
His four hundred years of studying magic made him a melting pot of crazy powers such as time stop. There are only five arguably six people in the show who would not immediately lose to that one power and it is just one of the few we know of. When he gets his hands on Fairy Heart, Zeref gets infinite magic power. At this point, he is gearing up to rewrite reality. He is/was basically Thanos with the infinity gauntlet but without the risk of exploding on use. Dragon-Slayers(and few of them to boot) were the only ones to stand a chance against him. This was only because of their abilities to eat/burn enemy magics.
5. Laxus or Gildarts : Laxus, Gildards and Gray are all debatable at this point but I personally believe that while Gray may rival the other two in power he is certainly outclassed skill-wise.
[The only non-gif pictures I could find of just the two of them were them kissing: REALY? Internet, I am astounded at you! Kinda…]
6. Weaker of Laxus or Gildarts: (See 5)
7. Gray:
Sorry but this story is more about Natsu than it is about you man.
I did not look for a picture of you. Sorry I was disillusioned with humanity by the time I finished with Guildarts and Laxus. Shudder…..