I gave some characters a stats system like the one from FF tactics (Guild book is the scource).

it goes (From worst to best):
None
D-
D
D+
C-
C
C+
B-
B
B+
A-
A
A+
S
Invincable/Infanate.

I know it's Du'h obvious what's worst or best but I just felt like listing them.