In top-to-bottom order:
Special Grade (S)
Advanced Grade (A)
Better Grade (B)
Common Grade (C)
(There are also potential grades with plus (+) or minus (-), these are known as transitional ranks.)
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Champion rank = Lv 80 above
Elite four rank = Lv 60-79
High Elite rank = Lv 55-59
Mid Elite rank = Lv 50-54
Low Elite rank = Lv 40-49
Advance rank = Lv 30-39
Rookie rank = Lv 01-29
(This ranking system also applies to trainers. A trainer with an average team level of 63 would be considered an elite-four level trainer, even if he has a Pokémon that's not yet reached said rank.)