From top to bottom, strongest to weakest:

SSS

SS

S

A

B

C

D

E

F

G

H

I

From https://danmachi.fandom.com/wiki/Falna

Basic Abilities (基本アビリティ Kihon Abiritī): The Basic Abilities are Strength (力 Chikara), Endurance (耐久 Taikyū), Dexterity (器用 Kiyō), Agility (敏捷 Binshō), and Magic (魔力 Maryoku). Each one is shown with a rank and a number from 0-999. 0-99 is I, 100-199 is H, 200-299 is G, 300-399 is F, 400-499 is E, 500-599 is D, 600-699 is C, 700-799 is B, 800-899 is A, and 900-999 is S.

Tanks

Based on Bell's evolution I assume 1000-1199 is SS and 1200-1599 is SSS.