The obvious answer is The Legends, but they are hard to get. If you can get it Alakazam, but otherwise Mr. Mime or even Kadabra
Alakazam has INSANE stats. Psychic hits like a truck on steiroids.
Mr. Mime has more raw power than all the psychics but Kazam, Kadabra, and the legends. It also has a not terrible movepool, with access to T-Bolt and SolarBeam.
Kadabra has more power and speed than Mr. Mime, but is also more frail. he has the worst movepool out of the three though.