Gardevoir deserves any trainer's full attention. She can be really strong or pathetically weak, depending on how she's treated. Here is the EV spread, IVs and stats and moveset I give ALL of my Gardevoirs when I play.
Ability: Synchronize: for passing on pesky status moves.
Modest Nature: in my opinion the only nature she can use right.
IVs: HP: 29 / Atk: 19 / Def: 27 / SpA: 31 / SpD: 30 / Spe: 31
EVs: 252 SpA / 100 SpD / 158 Spe
-Psychic / Psyshock: Depends on if you want to kill all physical defenders, or take down some special walls with bad Defense 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 Water/Ground Pokemon 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.