I mentioned before in my Magneton post a decent point. To paraphrase: "A Pokemon's viability is greatly enhanced if they have a variety of rules to fulfill, but it can be made up for by a One-Trick-Pony if that one trick is done like no other Pokemon." This is the reason why Magneton failed, but why Mega Medicham might succeed.
We're talking about a Pokemon that can LITERALLY 2HKO the metagame--yes, it can even 2HKO 252/252+ Hippowdon and Quagsire with Hi Jump Kick, while having a 99.6% chance to 2HKO 252/252+ Slowbro with Thunder Punch after Stealth Rock. If I may point out like everybody, that's insane. We're talking about a Pokemon that cannot be switched into very easily; you might get lucky with a Gengar switch on Hi Jump Kick, but what if they predict and use Zen Headbutt? What if they run Bullet Punch, which has a 93.4% to 2HKO after Stealth Rock? Intimidate users like Landorus and Gyarados cannot switch in safely, since the fourth move is a decent tossup between Ice Punch, Thunder Punch, and Bullet Punch (I'd argue that Zen Headbutt is a safe alternative when you need to bank on a move hitting in comparison to Hi Jump Kick sometimes). Of course, my opinion doesn't matter because I'm the one jackass that runs a SubPunch set and called a pleb, but I know the main selling points where others don't. I mean...comparing it to Rampardos and Tyrantrum is unfair; Rampardos has a significant speed problem and Tyrantrum is a set-up sweeper, completely different from Megacham's immediate wall breaking power. The closest comparison I can think of is Kyurem-B, but even that's a bit of a stretch because it has the ability to go mixed. Megacham is physical power all the way, a Pokemon no other would want to come into.
I'd argue that Mega Medicham has next to no counters, since there is no safe switch-in that can immediately threaten it without risking its own skin. Reuniclus is a great counter, since it can reliably switch-in, only risks a 3HKO, and can set up Calm Mind and Recovers easily. Tied with it, Mega Sableye is a very safe switch-in, even punishing Megacham for using Hi Jump Kick. Coming in as well is Mega Slowbro, which doesn't care about even Thunder Punch (it's also immune to Critical Hits!). Mew would be second, but risks a 2HKO after Stealth Rock, assuming the 252/0 spread.
No matter what I said, however, they are NOTHING compared to the ultimate stop to Mega Medicham: Cofagrigus.
Kill me. A Pokemon considered a gimmick is the best answer to this thing.
Not only does it prevent Hi Jump Kick spam, but merely touching it removes Pure Power (not to mention its access to Will-O-Wisp and Hex).
So...
We're talking about a Pokemon which is only hard stopped by four Pokemon, two of which are below OU and one of them barely sees the light of OU to begin with. That's pathetic.
Checking Megacham is still relatively difficult due to its coverage moves, which means scouting its moveset is necessary to find out what it's running. Without Ice Punch, Landorus-Therian can safely switch in and Intimidate it while setting up on its own. Without Thunder Punch, Slowbro has a field day crippling it with Scald. Without Zen Headbutt (I've seen people not use it), Clefable absolutely laughs at it. Pokemon like Gliscor and Gyarados seem like good switch-ins, but Zen Headbutt devastates them (hence why I like running the move). Hippowdon and Quagsire, as mentioned above, cannot switch into Hi Jump Kick, even when fully invested in Physical Defense and HP.
Revenge Killing Megacham is, unfortunately (or fortunately!), the best way to deal with it. Even with Bullet Punch being a thing on it, most Pokemon that can outspeed it threaten it. Talonflame is Revenge Killer Number One; it can either Brave Bird to kill it off or Wisp+Roost Stall to whittle its health away. Gengar outspeeds Megacham, only really risking a 2HKO after Stealth Rock from Bullet Punch while OHKOing easily with Shadow Ball. Keldeo deals massive damage with Hydro Pump, OHKOing easily with Specs and Life Orb. Latios has a 93.8% chance to OHKO with Draco Meteor after Stealth Rock while unboosted. Mega Lopunny's Fake Out + Return KOs. Mega Pinsir's Return does a hilariously high amount of damage. Choice Specs Raikou's Shadow Ball is a guaranteed OHKO. Serperior does 84.2% minimum with Leaf Storm. Starmie's Scald does 85.8% minimum. Thundurus has a 25% chance to OHKO with Thunderbolt after Stealth Rock. Tornadus-Therian can OHKO with Hurricane, but has to worry about shaky accuracy. Weavile's Icicle Crash does 74.7% minimum. In Sandstorm, Excadrill outspeeds and OHKOs with Earthquake without worry.
Keep in mind, however, that the above are merely Revenge Killers and all have extreme difficulty switching in. Basically, the best way to get rid of Megacham is to either run Reuniclus, Megabro, Megaeye, or Cofagrigus (why?) or be prepared to sacrifice a Pokemon in order to Revenge Kill it (which is also outrageous). It seems like this would be a centralizing Pokemon when those are the absolute safest ways to keep it off the field.
Despite all of the credit I give Megacham, it has its issues. For starters, Base Medicham has an awful speed, almost forcing it to run Fake Out in order to safely Mega Evolve. Even after that, Base 100 is good, but not fantastic, leaving it outsped by multiple Pokemon, particularly Choice Scarf Pokemon--even Choice Scarf Tyranitar is outspeeding and dealing 58.6% with Crunch. Pokemon like Ferrothorn and TankChomp guarantee that if they go down, the Revenge Killer will have an easier time taking it down, thanks to it being a Physical Attacker; the combination of Rocky Helmet and their respective abilities will leave Megacham with 70.88% HP remaining after the attack, which is just low enough for multiple Pokemon to Revenge Kill. This also doesn't consider Hi Jump Kick's accuracy and downside when it misses, though Lopunny has the same accuracy issue as well (Scrappy helps against Ghost-Types, though).
With people trying to find out Megacham's flaws in order to prevent a rise, why aren't people doing a similar thing with Mega Scizor? Mega Scizor is undoubtedly dangerous and powerful, but many would argue that there are Pokemon that can handle it even easier; multiple Pokemon carry Hidden Power Fire for it now, while its checks like Landorus-T, Quagsire, and Heatran stick around well. Both have issues, but they do their jobs outstandingly well.
Considering that Megacham's power is so unrivaled that all bar Landorus-T in all of A can switch in, and that there is no other Pokemon that can do its job as well as it can, I can safely say that I support Mega Medicham rising to A+. I probably missed a few points, but most good ones were used before.
Happy Battling.
Oh, and sorry this wasn't as intricate as my normal posting. Been working on side projects all day and wanted to make sure this was posted before it became irrelevant.