It doesnt realy matter if we move Sylveon up tbh, C+/B- at the end of the day it doesnt realy matter since both ranks are fine for it. I often find myself tempted to put Sylveon on one of my teams as a part of a defensive core cuz it beats just so much shit in this meta its unreal, AV Azu (takes a while but works), Thundy, non Sludgewave Lando-I, Kyurem-B, all 3 wall breakers, Greninja, Keldeo, Latis, every Dragon bar Zard X and more (note: it doesnt counter all of them, to some its only a check). When dismissing Healbell for HP Fire it can even dent most steel types bar Heatran. However, it has a major drawback and thats the reliance on Wish + Protect to heal. It totaly kills any momentum you might have if you switch it into something it wants to wall because you need 2+ turns to heal back up giving the opponent room to do what he wants. At the end of the day its high risk/high reward. It used to be B rank a few weeks ago and imo that was the maximum for it C+/B- is realy just a matter of preference.
If by three wallbreakers you mean the three wallbreaker megas you are wrong.
252 Atk Pure Power Mega Medicham High Jump Kick vs. 252 HP / 4 Def Sylveon: 250-294 (63.4 - 74.6%) -- guaranteed 2HKO after Leftovers recovery
252 Atk Pure Power Mega Medicham Zen Headbutt vs. 252 HP / 4 Def Sylveon: 307-363 (77.9 - 92.1%) -- guaranteed 2HKO after Leftovers recovery
Sylveon can't switch in to Mega Medicham at all. That's not a counter or a check. Even if it switches into a fake out it puts sylveon within range of being killed by a zen headbutt. Even if the sylveon is max defence with a defence boosting nature it is still 2HKOed by zen headbutt and takes a minimum of 42.1% from a high jump kick.
Gardevoir also wins 1v1 if it's the calm mind set and it sets up while sylveon is protecting to get a wish. Sylveon is closer to a Mega Gardevoir check but it still isn't.
Mega Heracross is also a pokemon which sylveon is unable to switch into even unboosted. If it's boosted sylveon is dead.
252+ Atk Mega Heracross Pin Missile (5 hits) vs. 252 HP / 4 Def Sylveon: 210-245 (53.2 - 62.1%) -- guaranteed 2HKO after Leftovers recovery
252+ Atk Mega Heracross Rock Blast (5 hits) vs. 252 HP / 4 Def Sylveon: 280-330 (71 - 83.7%) -- guaranteed 2HKO after Leftovers recovery
+2 252+ Atk Mega Heracross Pin Missile (5 hits) vs. 252 HP / 4 Def Sylveon: 415-490 (105.3 - 124.3%) -- guaranteed OHKO
And even if it's max defence bold:
+2 252+ Atk Mega Heracross Rock Blast (5 hits) vs. 252 HP / 252+ Def Sylveon: 370-440 (93.9 - 111.6%) -- approx. 87.5% chance to OHKO
So sylveon counters or checks none of these (except gardevoir in some situations). I'm not saying sylveon isn't worthy of a raise. Quite the opposite. But saying that it counters or checks things it doesn't is not a good point for it to be raised. In a later post I will actually nominate something.