Two sets I find most useful
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Modest Nature
EVs: 252 HP,48 Sp Atk, 176 Sp Def, 32 Spd
-Will o Wisp
-Wish
-Psychic
-Thunderbolt/Focus Blast/Shadow Ball
Wish Support is always nice, Will o Wisp stops physical sweepers switching in to drop Gardevoir as you wish. Last slot really is about who you want Gardevoir to hit more often. Gyarados/Tyranitar/Azelf
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Timid Nature
EVs: 56 HP, 252 Sp Atk, 200 Spd
-Calm Mind
-Psychic
-Thunderbolt/Focus Blast/Shadow Ball
-Will o Wisp/Wish
Lol, almost the exact same with a nature/ev change as well as adding Calm Mind to the must haves. Yes this is a Calm Mind Gardevoir, but she is quite effective one as well.
Nature should almost always be trace, but Synchronize is useable if you are happy with the IVs on one.
Speaking of IVs, pretty much make sure her HP is high, then its up to you about the others (personally I would make all the IVs high, except for Attack)
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Angelic Diablo's
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Slicer the level 46 Scyther!