252 Sp Attack 252 Speed, That is the best for Gardevoir. Gardevoir is mainly used as a sweeper so you want to strike hard and Be first to strike. Evs in Sp Defence is not good at all because of its poor HP stat. The moveset you give it must have moves with Sp Attack influenced in it. Ill give you a moveset for it:
Focus Blast-Dark type coverage
Will-O-Wisp-Burn the foe and lower its Attack
So this Moveset will be usefull because of Gardevoirs high Sp Attack.