I've been waiting since ORAS release for someone to bring it up but whatever.
After having used Mega Ampharos extensively since XY's release, (In fact that's pmuch the only Mega I used in my good teams) I believe Ampharos deserves to raise in B-/B, as his defensive sets checks/counters a lot today's Meta threats, here are some calcs on all the things from S rank to B- he can stomach hits from :
S Tier
+1 252 SpA Keldeo Secret Sword vs. 248 HP / 252+ Def Mega Ampharos: 144-171 (37.5 - 44.6%) -- guaranteed 3HKO
252+ Atk Mega Metagross Earthquake vs. 248 HP / 252+ Def Mega Ampharos: 180-214 (46.9 - 55.8%) -- 77.7% chance to 2HKO (If Megagross packs pursuit then Zen Headbutt is a 3HKO, Ice Punch is a threat to him though.)
+3 0 SpA Mega Sableye Shadow Ball vs. 248 HP / 0 SpD Mega Ampharos: 174-205 (45.4 - 53.5%) -- 37.1% chance to 2HKO
+4 0 SpA Mega Sableye Shadow Ball vs. 248 HP / 252+ SpD Mega Ampharos: 153-180 (39.9 - 46.9%) -- guaranteed 3HKO
A+ Tier
252+ Atk Bisharp Sucker Punch vs. 248 HP / 252+ Def Mega Ampharos: 97-115 (25.3 - 30%) -- guaranteed 4HKO
252 SpA Gengar Shadow Ball vs. 248 HP / 0 SpD Mega Ampharos: 121-144 (31.5 - 37.5%) -- 88.9% chance to 3HKO
252 SpA Gengar Shadow Ball vs. 248 HP / 252+ SpD Mega Ampharos: 88-105 (22.9 - 27.4%) -- 58.9% chance to 4HKO
252+ SpA Heatran Earth Power vs. 248 HP / 252+ SpD Mega Ampharos: 146-174 (38.1 - 45.4%) -- guaranteed 3HKO
252+ Atk Mega Lopunny High Jump Kick vs. 248 HP / 252+ Def Mega Ampharos: 169-199 (44.1 - 51.9%) -- 14.1% chance to 2HKO
+3 0 SpA Mega Slowbro Ice Beam vs. 248 HP / 0 SpD Mega Ampharos: 374-440 (97.6 - 114.8%) -- 81.3% chance to OHKO, something with Ice Beam and +3 Special Attack (uninvested yes maybe lets not talk about that) can't OHKO a Physically defensive Ampharos with Ice Beam, impressive, huh ? Needless to say CroBro cant do shit to Ampharos.
+4 0 SpA Mega Slowbro Ice Beam vs. 248 HP / 252+ SpD Mega Ampharos: 328-386 (85.6 - 100.7%) -- 6.3% chance to OHKO
+2 252+ Atk Sharp Beak Talonflame Brave Bird vs. 248 HP / 252+ Def Mega Ampharos: 131-154 (34.2 - 40.2%) -- guaranteed 3HKO
+2 252+ Atk Sharp Beak Talonflame Brave Bird vs. 248 HP / 0 Def Mega Ampharos: 180-213 (46.9 - 55.6%) -- 75% chance to 2HKO
252 SpA Life Orb Thundurus Hidden Power Ice vs. 248 HP / 0 SpD Mega Ampharos: 153-182 (39.9 - 47.5%) -- guaranteed 3HKO
252 SpA Life Orb Thundurus Hidden Power Ice vs. 248 HP / 252+ SpD Mega Ampharos: 114-135 (29.7 - 35.2%) -- 14% chance to 3HKO
A Tier
252+ SpA Life Orb Celebi Earth Power vs. 248 HP / 252+ SpD Mega Ampharos: 159-187 (41.5 - 48.8%) -- guaranteed 3HKO
252 SpA Mega Charizard Y Fire Blast vs. 248 HP / 0 SpD Mega Ampharos in Sun: 144-171 (37.5 - 44.6%) -- guaranteed 3HKO
0- Atk Mega Charizard Y Earthquake vs. 248 HP / 252+ Def Mega Ampharos: 94-112 (24.5 - 29.2%) -- 99.8% chance to 4HKO
252 SpA Mega Charizard Y Fire Blast vs. 248 HP / 252+ SpD Mega Ampharos in Sun: 106-126 (27.6 - 32.8%) -- guaranteed 4HKO
0- Atk Mega Charizard Y Earthquake vs. 248 HP / 0 Def Mega Ampharos: 128-152 (33.4 - 39.6%) -- guaranteed 3HKO
252 SpA Mega Charizard Y Dragon Pulse vs. 248 HP / 252+ SpD Mega Ampharos: 146-174 (38.1 - 45.4%) -- guaranteed 3HKO
252 SpA Mega Manectric Hidden Power Ice vs. 248 HP / 0 SpD Mega Ampharos: 124-148 (32.3 - 38.6%) -- 98.2% chance to 3HKO
0 SpA Rotom-W Hydro Pump vs. 248 HP / 0 SpD Mega Ampharos: 57-67 (14.8 - 17.4%) -- possible 6HKO
+2 252+ Atk Mega Scizor Knock Off vs. 248 HP / 252+ Def Mega Ampharos: 121-143 (31.5 - 37.3%) -- 85.9% chance to 3HKO
+2 44+ Atk Mega Scizor Knock Off vs. 248 HP / 0 Def Mega Ampharos: 145-171 (37.8 - 44.6%) -- guaranteed 3HKO
A- Tier
+1 192+ Atk Gyarados Earthquake vs. 248 HP / 0 Def Mega Ampharos: 320-378 (83.5 - 98.6%) -- guaranteed 2HKO
252 Atk Jirachi Ice Punch vs. 248 HP / 252+ Def Mega Ampharos: 96-114 (25 - 29.7%) -- guaranteed 4HKO
252 SpA Life Orb Starmie Ice Beam vs. 248 HP / 252+ SpD Mega Ampharos: 146-172 (38.1 - 44.9%) -- guaranteed 3HKO
4 SpA Starmie Ice Beam vs. 248 HP / 252 SpD Mega Ampharos: 98-116 (25.5 - 30.2%) -- guaranteed 4HKO
0 Atk Life Orb Tornadus-T Superpower vs. 248 HP / 0 Def Mega Ampharos: 108-127 (28.1 - 33.1%) -- guaranteed 4HKO
B+ Tier
252+ SpA Mega Alakazam Psychic vs. 248 HP / 252+ SpD Mega Ampharos: 136-162 (35.5 - 42.2%) -- guaranteed 3HKO
252+ Atk Adaptability Mega Beedrill U-turn vs. 248 HP / 252+ Def Mega Ampharos: 130-154 (33.9 - 40.2%) -- guaranteed 3HKO
252+ Atk Technician Breloom Bullet Seed (5 hits) vs. 248 HP / 252+ Def Mega Ampharos: 120-140 (31.3 - 36.5%) -- approx. 57.4% chance to 3HKO
252+ Atk Hawlucha High Jump Kick vs. 248 HP / 252+ Def Mega Ampharos: 129-153 (33.6 - 39.9%) -- guaranteed 3HKO
+2 252+ Atk Hawlucha High Jump Kick vs. 248 HP / 252+ Def Mega Ampharos: 256-303 (66.8 - 79.1%) -- guaranteed 2HKO
+2 252 SpA Raikou Hidden Power Ice vs. 248 HP / 252+ SpD Mega Ampharos: 162-192 (42.2 - 50.1%) -- 0.4% chance to 2HKO
B Tier
252+ Atk Conkeldurr Ice Punch vs. 248 HP / 252+ Def Mega Ampharos: 134-158 (34.9 - 41.2%) -- guaranteed 3HKO
252+ Atk Life Orb Adaptability Crawdaunt Crunch vs. 248 HP / 252+ Def Mega Ampharos: 164-195 (42.8 - 50.9%) -- 2.3% chance to 2HKO
+2 252 SpA Mega Houndoom Dark Pulse vs. 248 HP / 252+ SpD Mega Ampharos: 186-220 (48.5 - 57.4%) -- 94.1% chance to 2HKO
252+ SpA Choice Specs Omastar Hydro Pump vs. 248 HP / 252+ SpD Mega Ampharos in Rain: 137-162 (35.7 - 42.2%) -- guaranteed 3HKO
+2 252+ Atk Scizor Superpower vs. 248 HP / 0 Def Mega Ampharos: 275-324 (71.8 - 84.5%) -- guaranteed 2HKO
+6 4 SpA Suicune Scald vs. 248 HP / 252+ SpD Mega Ampharos: 106-126 (27.6 - 32.8%) -- guaranteed 4HKO
252+ Atk Strong Jaw Mega Sharpedo Crunch vs. 248 HP / 252+ Def Mega Ampharos: 159-187 (41.5 - 48.8%) -- guaranteed 3HKO
252+ Atk Choice Band Victini Zen Headbutt vs. 248 HP / 252+ Def Mega Ampharos: 126-148 (32.8 - 38.6%) -- 99.5% chance to 3HKO
+3 252+ SpA Volcarona Bug Buzz vs. 248 HP / 252+ SpD Mega Ampharos: 280-330 (73.1 - 86.1%) -- guaranteed 2HKO
B- Tier
252 SpA Life Orb Azelf Psyshock vs. 248 HP / 0 Def Mega Ampharos: 160-188 (41.7 - 49%) -- guaranteed 3HKO
+4 4 SpA Serperior Dragon Pulse vs. 248 HP / 252+ SpD Mega Ampharos: 196-232 (51.1 - 60.5%) -- guaranteed 2HKO
0 Atk Tangrowth Earthquake vs. 248 HP / 0 Def Mega Ampharos: 138-164 (36 - 42.8%) -- guaranteed 3HKO
As you probably noticed, the choice between Physically and Specially defensive most of the time comes down to what threats he handles better, but most of the time both sets can handle the same things.
But what can Ampharos do to theses threats in return ? Amphy has a good enough support movepool, he can Paralyze them with Discharge/TW, Pivot with his slow Volt Switch, attack with his powerful Dragon Pulse (any non-resistant offensive threat is 2HKOed by a non invested DP, that speaks for itself), he can even Poison M-Sableye thanks to Mold Breaker and then just stall it.
As for the "no recovery" problem, Ampharos can either rely on Wish Support to get healed, or sacrifice two moveslots for RestTalk, tbh Ampharos probably is one of the best RestTalkers in the tier given how good his type is and how he can absorb status aimed at him like its nothing. (Think Breloom, Venusaur, Sableye, most of the Prankster-TW users cant do shit to him.)
But wait, there's more ! Ampharos can forgo his purely defensive role and act as a Tanky attacker, able to OHKO/2HKO 75% of the meta (I'd do calcs if I had time, I guess you'll have to see for yourself, or to simply test it.), he can also as you may well know act as a powerful late game sweeper thanks to Agility allowing him to outspeed the 123 Base Speed Pokemons (iirc) and to swiftly end the game thanks to the excellent coverage offered by Thunderbolt/Dragon Pulse/Focus Miss, the Pokemons able to stop a +2 Ampharos aren't that numerous and surprisingly few teams are ready for it.
Now you're probably going to spout that hype slang that is "opportunity cost", Ampharos has a unique niche in checking a lot of the common threats in OU while acting as the slowest Volt Switch user, it needs a bit of support, but not as much as a lot of C Rank need (Come on, dont tell me Ampharos needs as much support as Camerupt, dont tell me he does not have more utility than, Bronzong.), let me remind you the definition of a C Rank Pokemon :
C Rank: Reserved for Pokemon that have notable niches in the OU metagame, but have just as many notable flaws that prevent them from being effective. Pokemon in the C tier often require significant support to be effective. Pokemon from this rank tend to face a lot of competition with the more commonly used Pokemon.
To me, the only competition Ampharos faces depend from his role : Defensive roles can be contested by Venusaur and Sableye, my point being that Venusaur and Sableye both take care of a different array of Pokemons (both cant deal with Char Y, for example.), so yeah the choice you'll do here probably depends of your teams needs, offensively speaking, that's something else, granted : The competition for the "Mega Special Attacker" is fierce, however, Ampharos still has his niche as a Bulky Attacker, more powerful than his Electric type breathen, Manectric, and bulkier than Altaria, one of the new Dragons.
With a really fucking solid typing to boast that takes care of an unique amount of threats, a good support movepool, powerful attacks, and diverse options, be it offensive or defensive one, I can't see clearly why Ampharos is deep down on the C Rank, oh right "
Pokemon in the C tier often require significant support to be effective." Nope. So let's see : Defensive sets pretty much need Wish Support to function without RestTalk, that's the sad truth, (I repeat myself : RestTalk Ampharos actually is really good.) however, I don't call that "significant", Wishpassers are a good option in Balanced teams and Stall (Ampharos wouldnt fit in HO anyway.) so that's not exactly a problem, especially since Ampharos works well with some wishpassers, such as Sylveon/Clefable and Alomomomomomololola, in fact, Ampharos works with many of the defensive Pokemons in the tier, such as Skarm and Slowbro. The offensive sets need support too, but not too much as well, as the support required depends of the set used : All Out Attacker probably needs hazard support/removal and/or wish pass to make his job correctly and keep him alive, Agility set just requires his counters to be out of the game (As in : any late game cleaner.) and has a lot of opportunity to grab a quick +2 Speed to end the game, while it may not be A material (or at least not in the current meta, i guess.), it is far from being C Rank as it doesn't require a lot of support : Most of the teams have Hazard removal/support and/or wish support so yeah right.
Also I'd like to rest my case by mentionning how good Ampharos is (again), this time by mentionning the archetypes he fits in : He makes for a great Rain Support by offering a slow pivot that can stomach Water, Grass and Electric attacks with relative ease, some people might prefer Mega Pert/Cross in rain, but I'd say Mega Ampharos has a more defensive role than Pert/Cross does and allows for good pivots into, say, Kingdra or Shell Smash Omastar, if Ampharos wants to take an offensive role, he can literally MURDER stuff with Thunder, that even has a nifty 30% Para to help him kill stuff. Anyway I just wanted to mention it, also Trick Room support, Ampharos is a really good Mega in TR, but he faces competition from Camerupt, but since both have their pros and their cons, I wont develop further that point.
Anyway I'm done, I just wanted to point the fact that Ampharos is really underestimated in the current Meta and that it deserves a raise so uh yeah,
Ampharos for B-/B.