Gardevoir

Charm + Shadow Ball and Synchronoise 

The addition of Charm significantly improves Gardevoir's overall performance, and threatens much of the meta with huge STAB damage without worrying about shields. The tradeoff is burden of having the worst energy generation in the game. 

A moveset that is crippled by gaining only 2 energy per turn (2 EPT) severely limits any use of charge moves, and removes any sort of shield pressure that would be possible otherwise. Shadow Ball is chosen due to its lesser energy cost than other options, in addition to its relatively neutral damage against most opponents, and coverage against Gardevoir's weakness to Ghost-types, Bronzong, and Metagross. The addition of Synchronoise, thanks to the Ralts Community Day, gives Gardevoir a cheaper charge move option energy-wise which makes it more viable than any of its other expensive choices. 

Charm + Shadow Ball and Return

Now that Ralts has been released as a Shadow Pokemon, Gardevoir has access to the normal-type move, Return. While it doesn't offer many advantages due to its lack of ability to deal super effective damage, it does have the ability to hit many targets for neutral damage and sports the lowest energy cost of any of Gardevoir's moves at the very reasonable 40, which means it will get more use when combined with Charm's low energy gains.

NOTE ON THIS SET: 

When purified, Gardevoir will automatically reach level 25, which has a bare minimum CP of 1919, meaning Gardevoir cannot use Return in the Great League currently.

Note on Psychic Vs. Synchronoise

In a September 2019 update, Psychic was buffed to 90 Power/55 Energy. This makes it slightly more expensive and slightly more damaging than Synchronoise, and also makes the two moves virtually interchangeable in most PVP settings. However, Sycnchronoise is still preferred when paired with Charm due to the latter's low energy gains..