In my opinion , the power ranking list should go like this :
1) Acnologia/Igneel.
2) DF Natsu
3) Zeref/WW
4) August/Dragon Irene
5) Gildarts
6) Human Irene/Erza/Laxus/Larcade
7) Brandish/Dimaria
8) Base Natsu
9) Gray/Gajeel.
I consider Erza's meteor feat to be an outlier. ( Similar to Thor's star level feat ).
And I feel since this is a shounen manga , at least DF Natsu should have surpassed everyone else in the series bar Acno and Igneel. DF was consistently portrayed as OP since the beginning of the series. Even if he isn't a match for the upper tiers without DF , I think with DF he should be above them . You know , shounen tropes.
And finally , my opinion regarding the two ( notorious ) PIS feats of Natsu :Against Zeref and Acnologia.
I don't consider either of them for powerscaling. Reasons , against Zeref , we saw that his guild mark was glowing at that time. That suggests he might have gotten a power up because he was inside the guild. Guild mark glowing represents he got power in a similar fashion to how the Tenrou team got back then against Acnologia. And similar to how the Tenrou tree used to boost the FT mages' MP. Fairy Tail has shown that it's capable of giving out power like this as it did on Tenrou . We don't know whether the same will work for Natsu if he's outside the guild.
Against Acnologia , well ... there isn't much I can say , but the fact that if the 7 Dragon Slayers really were capable of oneshotting Acnologia , then why doesn't Natsu just gather all of them and go oneshotting the god dragons in the same way ? He could end the quest in a single day. The fact he doesn't do that means the author doesn't consider the Acnologia fight canon. He doesn't want to consider that fight. He wants to retcon it. That's all I can say.