Please don't just read the bolded things and immediately write off my thoughts. Read the explanations behind my thinking so you can fully understand my argument :)
Main Discussion
A-->A+ Disagree
While Mega Charizard X set most of the common metagame trends, such as defensive Landorus-T, our good pal Zard-X seems to struggle just a little bit in the current metagame, and how it's shaping up. With Pokemon like Landorus-T, Keldeo, and Clefable (three Pokemon that can check/counter Zard-X) running rampant through the tier, our old friend Zard X seems to be struggling for air. According to the latest data statistics, Zard X is at about 5.1% usage, which is lower than Pokemon like Zard Y and Magnezone (who is only OU because of its niche imo). With Pokemon like LandoT and Clef with insane usage, Zard X struggles right now. It cannot accurately deal with Defensive LandoT or Clefable, both of which have a great chance of living a +1 Flare Blitz, with LandoT being able to OHKO with EQ and Clef being able to KO after the recoil from Flare Blitz. It all depends on what you believe makes a Pokemon viable, usage or how it performs in the metagame. Looking at both of those qualities, Zard X falls short. Regardless, Zard X is a strong Pokemon who should stay in A ranking.
A-->A+ Agree
Mega Lopunny is arguably the best Mega Evolution introduced in ORAS, and for good reason. With perfect neutral coverage (except when against Shedinja but f that) and extremely strong STAB attacks in Return and High Jump Kick, combined with the amazing offensive stats it has, how is Mega Lopunny not A+ rank yet? With access to priority in Fake Out (which is basically free damage) and coverage moves in Ice Punch and Thunder Punch, Mega Lopunny is one of the most offensive Pokemon in the tier. The usage stats reflect how good Mega Lopunny is, hitting ~7.9% usage. Sure Mega Lopunny has its checks like Talonflame and Mega Pinsir, but so do other A+ Pokemon like Mega Diancie, who is arguably a worse Mega than Lopunny.
(Mega)
A- | --> A Fenced
Mega Metagross is no doubt a strong Mega evolution, but I haven't really seen any reason for it to change ranking. Sure it has great stats and good coverage, but its easily checked by common Pokemon like LandoT and Heatran. While Mega Metagross is a disgustingly good Mega Evolution, it doesn't appear to either struggle or excel in the current metagame.
A- | --> B+ Disagree
Mega Latias is one of the premier users of CM in the tier, second only to Clefable and arguably Mega Slowbro. Mega Latias as a very good mix of bulk and offense, allowing it to nuke with Draco Meteor and also tank certain hits. Sure it has very obvious check and counters and is liable to being Pursuited, but so is Latios (which there is talk of making it S Rank), and that's why it should only reach A- viability. While underrated (and definitely underprepared for), it's BoltBeam defensive set can put dents in lots of common Pokemon, like Landorus-T, Tornadus-T, Azumarill, Skarmory and Keldeo.
A+ --> S Disagree
Latios just isn't as strong as Pokemon like Landorus-T who have a better chance of obtaining the coveted S rank. It's very predictable nowadays, either running LO Defog or Scarf/Specs. Thats it. That's all Latios can do. It doesn't have the unpredictability to function as an S rank Pokemon.
A+ --> S Disagree
Like Latios, Keldo is very limited in it's options of sets and uses, usually just acting as a play offensive Pokemon, running an offensive item, either LO, Scarf, or Specs. Keldeo has very limited uses past those.
Main Discussion
While Mega Charizard X set most of the common metagame trends, such as defensive Landorus-T, our good pal Zard-X seems to struggle just a little bit in the current metagame, and how it's shaping up. With Pokemon like Landorus-T, Keldeo, and Clefable (three Pokemon that can check/counter Zard-X) running rampant through the tier, our old friend Zard X seems to be struggling for air. According to the latest data statistics, Zard X is at about 5.1% usage, which is lower than Pokemon like Zard Y and Magnezone (who is only OU because of its niche imo). With Pokemon like LandoT and Clef with insane usage, Zard X struggles right now. It cannot accurately deal with Defensive LandoT or Clefable, both of which have a great chance of living a +1 Flare Blitz, with LandoT being able to OHKO with EQ and Clef being able to KO after the recoil from Flare Blitz. It all depends on what you believe makes a Pokemon viable, usage or how it performs in the metagame. Looking at both of those qualities, Zard X falls short. Regardless, Zard X is a strong Pokemon who should stay in A ranking.
Mega Lopunny is arguably the best Mega Evolution introduced in ORAS, and for good reason. With perfect neutral coverage (except when against Shedinja but f that) and extremely strong STAB attacks in Return and High Jump Kick, combined with the amazing offensive stats it has, how is Mega Lopunny not A+ rank yet? With access to priority in Fake Out (which is basically free damage) and coverage moves in Ice Punch and Thunder Punch, Mega Lopunny is one of the most offensive Pokemon in the tier. The usage stats reflect how good Mega Lopunny is, hitting ~7.9% usage. Sure Mega Lopunny has its checks like Talonflame and Mega Pinsir, but so do other A+ Pokemon like Mega Diancie, who is arguably a worse Mega than Lopunny.
(Mega)
Mega Metagross is no doubt a strong Mega evolution, but I haven't really seen any reason for it to change ranking. Sure it has great stats and good coverage, but its easily checked by common Pokemon like LandoT and Heatran. While Mega Metagross is a disgustingly good Mega Evolution, it doesn't appear to either struggle or excel in the current metagame.
Mega Latias is one of the premier users of CM in the tier, second only to Clefable and arguably Mega Slowbro. Mega Latias as a very good mix of bulk and offense, allowing it to nuke with Draco Meteor and also tank certain hits. Sure it has very obvious check and counters and is liable to being Pursuited, but so is Latios (which there is talk of making it S Rank), and that's why it should only reach A- viability. While underrated (and definitely underprepared for), it's BoltBeam defensive set can put dents in lots of common Pokemon, like Landorus-T, Tornadus-T, Azumarill, Skarmory and Keldeo.
Latios just isn't as strong as Pokemon like Landorus-T who have a better chance of obtaining the coveted S rank. It's very predictable nowadays, either running LO Defog or Scarf/Specs. Thats it. That's all Latios can do. It doesn't have the unpredictability to function as an S rank Pokemon.
Like Latios, Keldo is very limited in it's options of sets and uses, usually just acting as a play offensive Pokemon, running an offensive item, either LO, Scarf, or Specs. Keldeo has very limited uses past those.