Okay, first of all I'm assuming Mewtwo isn't an option. Other than that...define "the best Psychic". If you mean the one that can produce the strongest Psychic attacks, Kadabra's your pokémon; although Exeggutor does have a slightly higher Special stat, it's only partially Psychic, and back in RBY dual-types only got half STAB. Bear in mind, though, that practically every other choice of Psychic-type will be more durable than a Kadabra, possing both higher HP and higher Defense. Make sure your Kadabra learns Recover, although Recover won't be enough to save you in hardcore link battling. A Hypno would also be a good option, unless Speed is particularly important to you; Hypno's Special is only very slightly lower than Kadabra's and all of its other stats are higher save Speed.