Here's two sets, one for singles and one for doubles:
MegaGardevoir @ Gardevoirite
I'll explain the EVs in a sec
Modest/Timid
Pixilate
- Hyper Voice (Tutor Move)
- HP Ground/Psyshock
- Calm Mind
- Encore/Substitute/Pain Split (Tutor Move)
This is the singles set. Pixilate Hyper Voice is Gardevoir's strongest STAB, as it turns Fairy-type and gets a 30% boost in power. Also due to Hyper Voice and other Sound moves ignoring Substitutes now, there is no better option here. Ground provides the best coverage alongside Fairy-type you can manage, but Psyshock is a more available option for in-game purposes and it still hits Poison-types and special walls hard. Substitute works brilliantly alongside Calm Mind for protecting you against anything that might try to status you during your setup, while Encore can trap an enemy into an ineffective move giving you free setup turns, and since Gardevoir is immune to Dragon and quad-resists Fighting it has two strong options to lock enemies into.
Normally HP/Def training would be best for a CM user, but Gardevoir's lack of reliable healing options combined with abysmal Def will usually mean that setting up against a physical attacker will result in you being brutalized anyway unless you have Pain Split (but this may be worth a try.) Since MegaGardevoir has respectable Speed now, you can either play a pure-offense set with Timid and 252 EVs in Sp.Atk and Speed, or you can go a bulkier set with Modest and some split between HP, Sp.Atk and either defensive stat. If you find certain specific opponents causing Gardevoir problems with their attacks, try and tailor your EVs to defend and counter them.
As for doubles:
Gardevoir @ Focus Sash
Trace/Telepathy
Same EV rules as singles set
Modest
- Dazzling Gleam
- Ally Switch/Trick Room
- Trick Room/Imprison
- Protect
Doubles Gardevoir is a completely different animal to the singles set, and due to the superior (for doubles) abilities that Gardevoir gets over its Mega Evolution I'd honestly just use the regular Gardevoir in doubles and let someone else do the mega evolving. Either way Gardevoir's supporting options are numerous and that's what lets it shine, especially Ally Switch since it can tank Dragon-type attacks for an allied Dragon by swapping places with them on the battlefield. Trick Room is always reliable to have even on a team that isn't built around it, since it can completely shut down any opponent whose strategy relies on using fast Pokemon or boosting their speed (or lowering yours.) Finally, since most doubles Gardevoir would likely run similar sets, Imprison can completely ruin their day and even if they don't have Gardevoir, removing Trick Room and Protect from the equation as well as Dazzling Gleam on enemy Fairy-types to protect your Dragon-type partner can really be a bonus. The choice of ability depends on the team-mate; if they're likely to spam a move that hits every Pokemon on the field, Telepathy to avoid it is the obvious pick, if not then Trace can steal abilities like Intimidate and immunity abilities to disrupt opponent's strategies.