Regarding the justification for the first keys of these characters:
Natsu:
Town level (Comparable to
Gray, Defeated
Gajeel, Briefly contended with
Erza)
Gray:
Town level (Comparable to
Natsu.
Froze Juvia's Water Body while it was boiling and grew stronger since then; at his best,
produces that huge Ice Wall in the Oracion Seis Arc)
Gajeel:
Town level (Fought equally with
Natsu for an extended period of time)
Erza:
Town level with Standard Armor Set (Stronger than Base
Natsu and
Gray)
Juvia:
Town level (Damaged
Meredy, fought against
Gray on equal grounds)
There is a lot of circular reasoning going on here (Meredy gets her justifications from being able to fight Juvia), so in order to steamline these and remove circular justifications, shouldn't they be:
Natsu:
Town level (Comparable to
Gray)
Gray:
Town level (
Creates a huge Ice Wall in the Oracion Seis Arc)
Gajeel:
Town level (Fought equally with
Natsu for an extended period of time)
Erza:
Town level with Standard Armor Set (Stronger than Base
Natsu and
Gray)
Juvia:
Town level (Fought against
Gray on equal grounds)