In my opinion, either Alakazam or Gardevoir because they both are found in the beginning (although Ralts learns confusion fast, where as Abra doesn't learn a move until level 16 when it evolves into Kadabra) and have high special attack stats. Also if you play emerald alone and don't have anyone to trade with then definitely get Gardevoir because to get Alakazam you have to trade Kadabra which sometimes is hard to do/can't do until later in the game.
This is just my opinion.
Gardevoir. It has very high special attack.
-Azumarill -Magnetric -Breelom -(A flying pokemon) -(a normal, fighter or psychic)
You can buy it at the game corner.
Meditite is in Ruby, Sapphire, and Emerald. It’s Psychic/Fighting and evolves into Medicham starting at level 37.
Here is a list of all pokemon that can learn Psychic: http://www.serebii.net/attackdex/psychic.shtml
Gardevoir. It has very high special attack.
-Azumarill -Magnetric -Breelom -(A flying pokemon) -(a normal, fighter or psychic)
Gardevior psychic on tentacruel thunderbolt on the rest except whiscash use Breloom
You can buy it at the game corner.
He's fighting type? Use psychic or flying pokemon.
water, leaf, fighting, psychic, electric, there are a few good normal Pokemon too. if these are not available try and stick with electric and water
Meditite is in Ruby, Sapphire, and Emerald. It’s Psychic/Fighting and evolves into Medicham starting at level 37.
Tate & Liza in Mosdeep. Psychic type.
In Pokemon Sapphire, Ruby and Emerald the 7th Gym that's located in Mossdeep City has Psychic type Pokemon.
Use water pokemon, grass types or bug types because both Solrock and Lunatone are weak to these types
Here is a list of all pokemon that can learn Psychic: http://www.serebii.net/attackdex/psychic.shtml
Drowzee and Hypno are top choices. but most psychic Pokemon know/can learn Hypnosis.