personal opinion
1 Acnologia
2 Natsu (flames of emotion)
3 Zeref
4 August
5 Irene
6 Gildarts
7 God Serena
8 Larcade
9 Jellal on pair with Laxus
11 Gray
12 Erza
13 Gajeel ( could be higher if he was able to use dragon force at will)
14 Invel ( really underrated , if it wasn't for Juvia death he would have win against Gray)
15 Dimaria
I am gonna explain my choices :
On Acnologia absolutely no doubts ,he is clearly the strongest in FT.
I put Natsu second just because of the flames of emotions that he gets in the feat against Zeref where he actually defeats him. I didn't put the 7 flames of dragon slayers just because it is shared power , so i wouldn't count it as a Natsu ability .
Well Zeref in third isn't something to be explained , he is said to be the strongest mage ever.
August is the strongest of the spriggan 12 , defeats alone Crime Sorciere and could've destroyed Magnolia if it wasn't for Mavis.
Irene is definitely the 2nd strongest in the spriggan 12 , could've killed both Erza and Wendy , she is able to pick up a fight with Acnologia even if for a short time , she is praised even by Zeref.
I put Gildarts 6th because of his really good feats , he picks up a really good fight with August , completely destroys a weaker God Serena and he is considered as the strongest in the fairy tail guild for the most of the show.
God Serena is here because of 3 main reasons : he destroys the four of Ishgar alone , he is considered the strongest mage in Ishgar and he is cool.
Larcade could be higher because he is basically no match for everyone who had sex , he destroys Kagura but loses to Sting with Rogue powers.Still he is able to get up and could've killed Natsu if it wasn't for Zeref.
Well now it stars the difficult part :
In my very personal opinion Jellal is stronger than Laxus , but it's because i am a jellal fan . Well talking about their feats Laxus kills Wahl even if he was under ethernano poisonament , he has incredible body strength and stamina and gets a new form like Red Lightning Mode. For Jellal he one-shots Neinhart , gets destroyed by August , picks up a decent fight with Acnologia.He is also able to use like 10/11 types of magic and can outspedd basically everyone with Meteor.At last he never gets an upgrade form like the others ( I am kind of angry for this ).
Gray is really strong right now because of the DMS mode , that makes demons basically no match for him and makes his ice magic stronger: He defeats invel , but only to Juvia death , picks up a good fight with Natsu , but only because he is in END mode and he posses Iced shell , still if he isn't fighting a demon he can have serious problems.
Now i am kinda tired of writing , if u want me to explain the others 4 just ask