Gardevoir

The Embrace Pokemon has had an interesting history in Go. On its release, Gardevoir was primarily used to counter Fighting Types, in particular as a way to take down Machamp raids. It wasn't until much later that it was given Charm, allowing it to become the strongest Fairy-type attacker in the game. For players still early in the game, Gardevoir is a nice investment that can help open the door to many Tier 3 solo runs. For players that have stronger collections, it generally falls behind the big-name Dragons when fighting dragons, stronger Psychic-types against Fighting-types, and powerful Fighting-types when facing Dark-types. Still, it can fill holes against all three of these types. 

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 Gardevoir its trademark Fairy-type coverage.
• Psychic pairs with Confusion to give Gardevoir its strongest moveset Vs. Fighting Types, though it has much more competition as a Psychic Type.
• Shadow Ball can allow 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 Gardevoir do anything offensively.
• Synchronoise is inferior to Psychic. 
• Return has no use at all on 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. 
• Magical Leaf is a good move on defense, but it doesn't do many favors for Gardevoir.
• Triple Axel has wide neutral coverage and is outstanding on defense, though it leaves Gardevoir very vulnerable to Steel Types.
• 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.
• Return is three-bar move that comes out fast, but its low power renders it less than useful, and Gardevoir's main niche on defense is its coverage/resistances.

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 low energy per turn 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. 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.