At this point, being a dragon or dragon slaying magic is the strongest. Too many OP characters filling in the ranks. Seriously, the top....12 characters in the series all use it.
1) Acnologia
2) Igneel
3) Ignia
4) Mercphobia
5) Aldoron
6) Selene
7) Viernes
8) Grandeeney
9) Metalicana
10) Weisslogia
11) Skiadrum
12) Natsu
The dragon kings/god dragons (Acnologia, Igneel, Ignia, Mercphobia, Aldoron, Selene, Viernes) are all leagues ahead of every other character in the series combined. The parent dragons (Grandeeney, Metallicana, Weisslogia, Skiadrum) can wipe out a continental-level super weapon that can solo every mage on an entire continent, which is something that Makarov considers using Fairy Heart against. No other characters operates on such a scale. Natsu can beat Zeref with Fairy Heart. Feed him power from a god-dragon, and he's above even that and jumps all the way above the parent dragons. Also, Irene is a dragon slayer and tied with August for the strongest Spriggan. Laxus is also a DS, and he's one of the top dogs in Fairy Tail. Too many, too stronk.
Also, Erza has now joined the ranks of dragon slayers with her new sword. So yeah.