Well Ralts isn't now a pure Psychic, even though it was prior to the introduction of Fairy-type and Gen VI. Not that I hate Ralts, if I hate it I wouldn't have at least 6 Gardvoirs and 4 Gallade.
I just don't think a dual type should fit in a role for a pure type club.
The Abra line on the other hand is classic, has been a signature Pokémon of Sabrina, long ago before Gardevoir made her appearance to the franchise, let alone becoming a signature Pokémom formerly by Wally and now Diantha.
But among the suggested Pokémon Mewtwo wins by miles, since it is the strongest Psychic-type Pokémon! :)
How's your Monotype challenge going by the way?
Speaking of Monotype Challenge, our Psychic Club's relatively successful (thanks to everyone, I'm really grateful for that), that's why this section's moderator
Wiicked (his soon username) and I are making it PokéClub section wide! If you're interested in representing Psychic-type Club, please let me know!
Here's the planned details:
https://www.pokecommunity.com/posts/8926198/
Don't forget Ketsu's suggestion as well, please let him know whether you're supportive or not. So that we'll at least know if it's worth organizing! Any feedbacks and questions ar welcome!