282 Gardevoir

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 the burden of having the worst energy generation in the game. Confusion offers superior energy gain and solid damage output, but other Pokemon are better suited for a Psychic-type role, as Gardevoir is primarily used for its Fairy role. 

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. 

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, Synchronoise is still preferred when paired with Charm due to the latter's low energy gains..

Charm makes Gardevoir an anti-meta specialist, hitting popular Fighting, Dark, and Dragon-types extremely hard. Thanks to Gardevoir's Psychic-typing, it also enjoys wins against most other Psychic-types due its resistance in the Psychic mirror, while still hitting back neutrally with Charm. A high ATK stat distribution holds Gardevoir back significantly due to the resulting poor bulk. The prevalence of Steel-types in the meta also give Gardevoir a very difficult time. Overall Gardevoir is alright for its Fairy-type role, but more durable competitors like Wigglytuff tend to perform better. 

While Fairy-types are highly valued in Ultra League due to the dominance of Giratina, Gardevoir is unfortunate to have a Psychic sub-typing, which makes it vulnerable to Giratina's attacks. Beating up Fighting-types and comfortably winning against most Dragons in this league are definitely good perks, but losses against Giratina are big liabilities in the Fairy-type role. Popular Steel-type picks also give Gardevoir a lot of trouble. If you're considering a Fairy, bulkier Charm-users like Wigglytuff would be better choices. 

Gardevoir has many key matchups in the Master League due to its typing and great moveset. Access to Charm enables quick wins against popular Fighting, Dark, and Dragon-types not named Dialga or Giratina. However, losses to common Steel-types in this league really makes it difficult to use Gardevoir comfortably. As should be obvious by now, Gardevoir is a good Pokemon on paper, but it's held back due to being countered by so many of the top players in the league. At least its high ATK stat is unrestricted compared to previous leagues.