>>23389942Not him, but Dedenne might work if it get really lucky like that one Pachirisu, but that's it.
In OU Sylveon and Gardevoir has pretty much different roles. That said, Sylveon lack speed and Gardevoir has a bigger movepool. It must also be forced to use Choice specs or Life orb to use stronger Hyper voices. Sylveon has better bulk, but that's pretty much what I can think of.
And I hate when people keep saying Gardevoir will always get OHKOed, it's not true. Stuffs like Garchomp aren't even guaranteed to KO in Adamant nature.
252+ Atk Garchomp Earthquake vs. 0 HP / 24 Def Gardevoir: 246-291 (88.8 - 105%) -- 31.3% chance to OHKO
252 SpA Choice Specs Keldeo Hydro Pump vs. 0 HP / 0 SpD Gardevoir: 237-280 (85.5 - 101%) -- 12.5% chance to OHKO
252+ Atk Mold Breaker Mega Gyarados Waterfall vs. 0 HP / 24 Def Gardevoir: 225-265 (81.2 - 95.6%) -- guaranteed 2HKO
(Gardevoir use 24 defense EV so it doesn't get 2HKO by Latios's Psyshock)