As an avid Mega Lopunny player, probably used it more than anyone seeing as it's the only mega I've used since ORAS' release. . . I can't say I totally agree with Mega Lopunny being S. Yes, it can play an immense amount of roles, something nothing else can do, a huge feat mind you. However, I can't sit here and honestly say it's on the same level as Greninja, Megagross, Latios, Thundy, and even more so Lant. The reason is, it requires a bit of support if it isn't the support mon itself which can limit its offensive roles, something all of these really don't have problems with, and if they do, it's minimal, and they have insane power to outweigh it.
It's power isn't the greatest, I'll admit, and it needs PuP if it ever plans on 2HKO'ing the OU tier (which limits its support to an extent). Which is difficult to do anyways with such a balanced/bulky offense driven meta game right now (from what I've seen). There are just so many things that it has trouble with face to face, Physically oriented fairies, Priority users (outside of Sucker Punch), Scarfed 'mons, Stupid Lant, physical walls, etc. It does have an amazing support movepool, one of the best in OU I must admit, so I can see it going to S if it goes for being an offensive support 'mon.
However, the problem I'm seeing with that is, we'll end up with an inflated S rank if we plan on pushing Pokemon capable of pulling this off, as it would then mean we'd push mAlt, ZardX, Gard, mSableye, Heatran, and Talonflame as well. All of which, arguably, have some equally awesome sets they can run.
I won't say I'm fully against it going to S, but I just don't see it as S worthy. It's a great Pokemon my absolute favorite in fact, but I believe A+ is good as of now.