Shadow Gardevoir

Shadow Gardevoir is coveted for its high Fairy Type power, which actually puts Fairy on the map to a large degree. In Dragon raids in particular, it can be a surprisingly bulky counter that hits very hard, and is generally a top counter in Cloudy weather. However, when going head-to-head with Pokemon of other types that counter the Dragon, Dark, or Fighting types that Fairy is used for, it tends to fall a bit short. It's worth raising a high-IV Shadow Gardevoir, but it's most likely going to become a counter reserved for very specific scenarios.

Best PvE Offensive Moveset

• Confusion is slightly stronger than Charm in a neutral scenario, and both are clearly superior to Charge Beam and Magical Leaf, having better damage output and coverage.
• Dazzling Gleam pairs with Charm to give Shadow Gardevoir its trademark Fairy-type coverage.
• Psychic pairs with Confusion to give Shadow Gardevoir its strongest moveset Vs. Fighting Types, though it has much more competition as a Psychic Type.
• Shadow Ball can allow Shdow Gardevoir to attack into resisted Psychic-type Pokemon, but other Pokemon handle this role readily.
• Triple Axel is a good Ice Type attack, but it doesn't really help Shadow Gardevoir do anything offensively.
• Synchronoise is inferior to Psychic. 
• Return has no use at all on Shadow Gardevoir.

Best PvE Defensive Moveset

• Charm has more meaningful coverage against common attackers and hits hard with STAB. 
• Confusion is an excellent move on defense, but doesn't have as much coverage. 
• Dazzling Gleam threatens Dark-type counters while Shadow Ball hits Steel and Fire-types better. Neither are particularly hard to dodge.
• Psychic provides powerful, 2 bar STAB, though its coverage is a bit lackluster.
• Charge Beam is underwhelming in every aspect.
• Synchronoise is the hardest to dodge, but it falls behind Psychic.

Charm 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 lowseverely 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 Shadow Gardevoir's weakness to Ghost-types. Even lower-cost than that is Triple Axel, which grants an Attack boost on use that also increases the power of Charm for an added bonus. Synchronoise* hits relatively hard thanks to STAB and has a fairly low energy cost, but doesn't offer a whole lot coverage-wise.