700 Sylveon

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. 

The addition of Psyshock following the second Eevee Community Day is a very beneficial gain for Sylveon, giving it a Charge Move that is faster to access while also providing coverage against its weakness to Poison-types. Thus Sylveon's performance increases to be more competitive with the likes of Clefable, but falls slightly behind Alolan Ninetales due to the latter's faster Charge Moves and additional Ice-type coverage. 

Sylveon's access to Psyshock helps it compete against not only other Charmers but also the overall meta. The perk of having more stats than other Pokemon allows Sylveon to reach the CP cap of Ultra League in fewer levels, which negates any need for XL candy and results in a much more cost-effective candidate compared to other Charmers. 

The combination of Fairy typing, decent stats, and Charm is proven time and time again to be a successful formula in the Master League, as evidenced by Togekiss's continued dominance. Sylveon follows in these footsteps, although it is often outclassed by its Flying counterpart due to having slightly less stats and no Ground-type resistance. While Sylveon has  advantages over Togekiss, such as the lack of Ice-type weakness, and access to a more reliable nuke in Moonblast, it is often difficult to justify choosing Sylveon over Togekiss.

Nevertheless, the Master League isn't particularly great at handling Charm (or Fairy types as a whole really), so what better way to abuse this than to run two of them? Sylveon often sees play alongside Togekiss to really take advantage of Charm's oppressive pressure in a format dominated by Dragon types. Anchoring these two is a Pokemon that can simultaneously cover Steels, Kyogre, Ho-Oh, and Mewtwo in one slot - this Pokemon usually being either forme of Giratina. Giratina, in turn, appreciates the Charmers' ability to defeat the Dragon and Dark-types that plague it so much.