Gardevoir deserves any trainers full attention. She can be really or pathetically weak, depending on how she's treated. Here is the EV spread, IV's and stats and moveset I give ALL of my Gardevoirs when I play.
ability: Syncronize: for passing on pesky status moves.
nature: modest- in my opinion the only nature she can use right.
IV's: hp:29, atk:19, def:27, spa: 31, spd: 30, spe: 31
EV's: spa: 252, spd:100, spe: 158
Psychic/Psyshock: Depends on if you want to kill all physical defenders, or take down some special walls with bad def. like Snorlax.
Thunderbolt: Great power, good for water and flying types.
Energy Ball/Shadow Ball: Depends on whether you are wanting to take down Ghosts or mudfish like Swampert, which can get annoying.
Signal Beam/Focus Blast/Icy Wind: Yeah, I know. No Calm Mind. I think if her special attack is high enough, why raise it? Besides, Signal Beam is great against Grass, Psychic, and the dreaded Dark type. Focus Blast is also good for Darks, but is pretty inaccurate. I'd rather go with Icy Wind. But its your choice.