All right, mega gardevoir in B+
Really.
Mega Gardevoir needs to be A- at the VERY least, if not flat out A.
Her Hyper Voice OHKO's several frail pokemon and 2HKO's many many more. I actually use Will-o-wisp on her along with 3 attacking moves so i can cripple bisharp as it tries to sucker punch, which is really cool, same with breloom. It's hilarious when scizor SD in your face as they get burned XD.
But Mega Gardevoir's great performance against baton pass and overall power, coupled with decent bulk to make it an emergency check to the likes of keldeo, thundurus, greninja, and latios is great. She has no access to priority, and that's a tad frustrating i agree, but her raw power more or less makes up for it imo.
Her STAB combo is great too. Fairy/Psychic covers every type except fire and steel, and focus blast hits steel, so that leaves some frail fire types that barely stomach psyshock (talon, char-y) and an aegislash, which remains your most solid counter. Aegislash is super annoying to deal with, and her speed is a little dissappointing, which is why i'm not nomming mega garde for A+ or anything.
Taunt also screws with stall pretty well :] And trace lets you copy speed boost from scolipede, i've won a few matches against baton pass w/zapdos over mr. mime just because of that extra speed.
She can easily pair with the likes of bisharp or keldeo to remove each other's checks and stuff and form a pretty decent offensive core too.
So, definition of A pokemon:
Reserved for Pokemon that are fantastic in the OU metagame, and can sweep, support, or wall significant portions of the metagame. These Pokemon require less support than most others to be used effectively and have few flaws that can easily be overlooked when compared to their positive traits.
Mega Gardevoir is indeed fantastic in the current meta, and is more or less unparalleled in its role as a powerful fairy type special attacker, basically only receiving competition from specs sylveon. Gardevoir really only needs aegislash and super bulky shit like chansey gone and its guaranteed to poke big holes in the opposing teams as long as focus blasts hit :P Her flaws, mediocre speed and REALLY low physical bulk (and thus suspectibility to priority) are sorta annoying, but her decent special bulk can make up for that.
Mega Garde is a super powerful special wallbreaker which mainly uses a type with great offensive coverage to start with. When you have the second Stab removing a common type that walls your first, there's a sign of a decent pokemon already, and mega gardevoir is just that. (that's what makes fire/dragon STABs so fantastic)
So yeah, get this lady out of B+