This is the system that I will be using.
Beginner - F (baby Pokémon)
Novice - D (starters)
Average - C (trainer who trained their Pokémon though don't reach the level of elite)
Elite - B (Pokemon ranger, veteran trainer, etc…)
Gym leader - A (adult misty, Koga, etc…)
Elite four - S (Lorelei, Bruno, Agatha, etc…)
Champion - SS (Adult Lance, Adult Cynthia, etc…)
Legendary/Master - SSS (Ho-ho, Mewtwo, etc…)
God - EX (Arceus, Mew)