Wow, didn't even know Garde is only in A Rank, which is downright absurd. Musharna has insane bulk, but Gardevoir has usable speed and incredible versatility on her side. Trace is also pretty awesome for a situational ability, revenge kill weather sweepers, gain unexpected immunties/resistances, shrug off status or damage etc. Gardevoir can do damn well anything you want it to. Calm Mind with its usable speed ensures it isn't outclassed by Mushy. Perhaps you want to hit hard right off the bat? Choice sets or Life Orb are fine options, while Mushy needs Trick Room to sweep like Garde does. Garde has one of the largest support movepools I've seen in a Pokemon (see above). You can't simply switch in your Psychic check in on Garde without hesitation, especialy if you don't know its set. A well played Gardevoir can easily dismantle teams, if not help a fellow teammate finish the job with Dual Screens, Memento or Healing Wish.
Gardevoir for S-Rank, I'm surprised she wasn't already. She has very few exploitable flaws (inability to boost speed) and even then, her vast movepool, sheer unpredictability and brilliantly placed stats more than makes up for it.