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.
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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