Get ready for a wall of text
The reason I give a slight edge to Natsu is for a few reasons
1. I believe Zeref with Fairy Tail is stronger than Normal Dragon Acnologia, by the smallest incrament, because he has two large magic sources that surpass Dragon Cry, and his Hax is incredibly Broken, and Savage Dragon Fire Natsu Beat Him, so already Savage Dragon Fire Natsu is stronger than Normal Dragon Acnologia
2. In the final Battle with Acnologia, Savage Dragon Fire Natsu, took a full powered explosion from Human Acnologia with the Space Between Time, and took no damage, which surprised Acnologia, which already puts Savage Dragon Fire Natsu close to Human Acnologia with the Space Between Time, not stronger but close
links to Natsu tanking Explosion:
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3. After that, Natsu then stacks Seven Flames Dragon Mode ontop of Savage Dragon fire, and belives he has enough power to defeat Human Acnologia with SBT, however Acnologia gets cocky and says they will never beat him, which is probably because of his Resistance to Magic, I think in that moment, SFD Natsu was stronger, but Acnologia's Resistance would have made the damage weak enough, that it wouldn't kill him.
4. After that, something happened that I never noticed before, Natsu stacked Dragon Force on Top of SFDM, and SDF, making him at the very least 3 times stronger, It's been established that Dragon Force gives a Dragon Slayer the true power of a Dragon, so it stands to reason that a Dragon Form, should be a comparable increase to Dragon Force, (as well the argument could be made that Human Acnologia has better increase in power with the SBT, than his Dragon Force, as Human Acnologia is surrounded by the Magic) and since SFD Natsu is already a decent amount stronger than Human Acnologia with SBT, and then stacks DF on top of that, he should be by the tiniest Margin ever, Slightly strionger than Dragom Acnologia with SBT, which would make him the strongest character, NOT TO MENTION HE ONE SHOT ACNOLOGIA, EVEN WHEN HE HAD MAGIC RESISTANCE
This is my opinion, I didn't believe he was stronger until I realized he could stack Dragon Force ontop of Seven Flames Dragon, which made me believe he stronger, I do believe It could be the other way and Acnologia is still stronger than him, however I belive Natsu's chances of being stronger are good
this is the scaling chain
Human Acnologia=<Base Zeref<DF+FDKM Natsu<<<Dragon Acnologia=<Fairy Heart Zeref<Savage Dagon Fire Natsu<Human Acnologia SBT<Seven Flames Natsu<Dragon Acnologia SBT=<SFD+DF Natsu