Ralts is a pretty good Psychic type. It just requires a little bit of babying before it gets some good attacks, as Abra was like before.
For the starters, you should look up their final evolutions to see which one you like the most. Blaziken, Torchic's final form, is a beast and is Fire/Fighting. Swampert, Mudkip's final evolution, is Water/Ground and is great as well. Last, there's Sceptile, Treecko's final stage, and it's a Grass-type. I prefer either Blaziken or Swampert due to their other typings, so I have no real opinions on Sceptile.
Really, just look up an online Pokedex for Ruby/Sapphire/Emerald and see all the different Pokemon available to you. Pick whichever ones look cool, seem strong, or have attacks that you want to use.