Psychic being the best offensive type is kind of exaggerated, but it's still a great type. The main thing is both of the Uber Pokemon (Mewtwo and Mew) are Psychic. Psychic is also only resisted by one type (Psychic, lol) and the main Psychic move (Psychic, LOL) lowers Special 30% of the time, so the damage piles up.
Is Psychic the best offensive type? Psychic
Pokemon are some of the best, but Ice, Rock, Ground, Flying, etc are much better since they're effective against more types. Psychic is also only super-effective against Fighting and Poison, and in OU, the only Pokemon of those two types that is really viable is Gengar. Ice on the other hand is super-effective against Flying, Dragon, Grass and Ground, and only resisted by Water and Ice. It's a much better offensive type.
Defensively, again Psychic
Pokemon happen to be some of the best, but Psychic type itself kind of bites. It resists other Psychics, Fighting (lol), immunity to Lick, and that's it. Sure, it doesn't really have any weaknesses, but Psychics don't really resist anything either. A much better defensive type is Ghost which has two immunities, one resistance and only one basically non-existent weakness, but unfortunately, the Ghosts are all Poison. Water is also great having three resistances and only two weaknesses
Is Psychic type the Kirby of RBY? I'd say so; that's a really good analogy. Extremely user-friendly, low-risk, high-reward, but you need a combination of types to do well and an all or mostly Psychic team is super easy to stall against. My main team consists of three Psychics and three Normals.
Anyway, this is getting a bit off-topic, haha, but if any of you are interested, I have a 200-page Advanced Battling guide on GameFAQs all about it. Or if anyone wants to PM me any questions, I'll be happy to answer! :D
It's impossible to have a Mewtwo at Level 255 due to his experience growth type. The max is 237.