After you have beaten the Pokémon League, go back to Cerulean City. Northwest of Cerulean City in a cave is Mewtwo, the best Pokémon there is
(# 150). He is at Level 70 and is at the end of a long way in the cave. There are Level 50s Pokémon from everywhere (Venomoths, Chanseys, Kadabras, Raichus(!!!), Magnitons, Marowaks, Hypnos, etc.). It is possible to catch Mewtwo without the Master Ball, but it is hard. He is no pushover and will do a lot of damage against you. He has Swift, Psychic, Recover, and Barrier at his command. Which means 2 attacks (one fast and one powerful) and two defenses (one for healing and one for defense), making
him a balanced and powerful Pokémon.
