Sylveon

The Generation 6 lone Eeveelution was one of the first Fairy Types revealed in the entire series. In fact, it was revealed before Fairy Type itself was revealed, leading to substantial confusion at that time! But trivia aside, how does it perform in Pokemon Go? Not very well. Its attack stat is too low to be a worthwhile attacker, though it's a decent budget option for Fairy Type damage. Earlier in Pokemon Go's life, Sylveon likely would have been a staple Pokemon similar to Vaporeon, but that time is long over.

Best PvE Offensive Moveset

• Charm is the fast move of choice, as Quick Attack is basically worthless on offense.
• Dazzling Gleam is the charged move of choice, providing maximum DPS with reasonable energy cost.
• Mooblast is less reliable, and generally inferior.
• Draining Kiss is inferior.
• Psyshock* lacks STAB for Sylveon. 

Best PvE Defensive Moveset

• Charm is substantially more powerful than Quick Attack, but at least the former offers off-type coverage.
• Psyshock is recommended for defense due to its coverage against Poison-type attackers, as well as its 3-bar move accessibility. 
• Dazzling Gleam hits hardest and comes out frequently enough to be reliable.
• Moonblast takes too long to use, and Draining Kiss is weaker.

Sylveon only really has one viable moveset, made up of its only moves that aren't bad. Charm provides strong STAB fast move damage that isn't widely resisted. Quick Attack trades damage for greater energy gains, but lacks STAB and can't be Super Effective against anything. 

Moonblast is stronger than either of Sylveon's other Fairy-type charged moves. Psyshock from the second Eevee Community Day event is the most optimal coverage move for Sylveon, giving it an option to fight back against Poison-types while costing less energy than Last Resort.