MGarde defenitely deserves A- IMO, the amount of damage it causes to Stall and defensive teams in general is unreal. Yes, it's completely eaten alive by the most centralising mon in the tier, but then again, so are the Latis. Aegi isn't what's stopping MGardevoir from being A-, it's what's stopping it from being A+. Pairing up MGarde with Pursuit Bisharp or Mandibuzz+Amoonguss can be really effective, sine once Aegi is taken care of, MGarde doesn't really need to worry about much else. The only other real counter is Mega-Scizor, and even that's pretty shaky since a lot of people have actully been using Will-O-Wisp which completely screws it.
Sure, it's not great vs Offense, but it can find a few opportunities to come in against the likes of Latias and choice-locked Terrakion, and once it does, you can pretty much kiss goodbye to one of your mons. Aegi isn't what's stopping MGardevoir from being A-, it's what's stopping it from being A+.
In a similar vein, I'd like to suggest Mega-Medicham for A-. It pretty much has the same qualities as Mega-Gardevoir : eats stall alive, hits incredibly hard, guaratees a KO vs offense if it somehow comes in for free, etc... Probably not quite as good as MGarde given that it has a few counters (buly psychics, particularly Slowbro who is often seen on stall and stops MMedi from 6-0ing) and his main STAB , HJK, isn't nearly as spammable and has bad accuracy and a terrible downside if it does miss (And if you're thinking of using Drain Punch instead, please don't. Unless you enjoy losing to Stall, of course.) However, it makes up for it by hitting a lot harder than MGarde, and that's saying something.
It's also an amazing Substitute user, due to how many switches it causes, and actually can kinda get past Aegi if it chooses to tun Fire Punch. Also, it it helped out by Pursuit support a huge amount, since everything that can take its STABs absolutely loathe it.
Also, and this was suggested a few pages ago, but I support Suicune moving up. So many teams just have nothing for Suicune at all, in fact, (and I don't know if it's just me), but most of my teams just auto-lose to it. Sure it's a little inconsistent due to Sleep Talk rolls, but once everything that can take it out during one Rest cycle is gone, (and not all that much can), than it basically wins bar crits. Someone actually asked it to be compared to other B+ ranks, but honestly, I feel it more comparable to Clefable, at least the Crocune and CM Magic Guard sets respectively : both are really solid win conditions with great defensive typings and are "immune" to status or at least not stopped by it, but Suicune has more overall bulk in excange for a buch of turns where it it vunerable and has to rely on luck, and only has one offensive move to use, so is not quite as good overral. Still, it's a pretty solid win condition and Scald burns can make it very difficult to break. I've also heard of an offensive CM set, but haven't actually used it so I can't really comment on it.