Mega Altaria: S --> A+: Neutral
The main thing with M-Altaria is 4 slot move syndrome. It hits like a truck even at neutral to many things, but one popular physical wall, (other than Garchomp, but even Rough Skin+Rocky Helmet does well) and it finds itself struggling. Lando-T and Ferro give it tons of trouble, and a burn will cripple it unless it's running Facade, not too common if you ask me. That being said it still does deserve to stay because without any of the aforementioned checks, Scizor, or Weavile (or Bronzong, this thing runs rampant over teams. It can run Fire Blast or special sets, but those don't quite have the popularity or prowess of the DD set (in my opinion).
Mega Gyarados: A+ --> A: Disagree
Mega Gyarados... Down to A... WHAT? Has NOBODY used this thing properly? It's a god damn MONSTER. It sets up a Dragon Dance, now what? It's incredibly bulky, it has a great type offensively (and it's not even THAT bad defensively), It can beat MZard X one on one, it OHKOs or 2HKOs almost the entire metagame, Ferrothorn, who may I remind you is SUPPOSED to be one of it's counters, has a high chance to be 2HKO'd by a +1 Crunch after SRs, can break stall with Taunt, avoid status with Sub DD, etc. WHY ARE WE TRYING TO LOWER THIS THING?
TheNegativeOne's post explains this better than I could ever.
Heatran: A+ --> A: Nein, Disagree
Far too splashable on most teams. Walls many threats. Only thing keeping it from being S-Rank (imo) is lack of reliable recovery and weakness to some common attacking moves/types.
Weavile: A --> A+: Agree
Can outspeed and OHKO so many common threats and 2HKO's almost all common defensive walls, has very few switch-ins, and is a danger to most OU mons in general. However, priority is its bane; its too frail to switch in on anything other than a well predicted Psyshock. That being said, it's still too good and too powerful not to be A+. It and Bisharp do similar-ish things (Knock Off 4 dayz), but if Bisharp is A+, then there's no reason Weavile shouldn't also.
Starmie: A --> A-: More inclined to agree, but still not sure...
This opinion is more a lack of experience using Starmie than anything else. I found it relatively successful with two sets, AV (which is garbage now btw), and LO spinner. LO spinner was the more successful of the two because it did the two things I asked of it well: spin, and outspeed stuff. I've never properly used Reflect Type, or been able to slap recover on any of my sets. The reason i'm still unsure is that, well, it did its job. Granted, it wasn't a very taxing job, but it did well. What keeps me still on the fence is that is usually doesn't hit hard enough, or gets walled by Ferro, some Scizors, etc.
Mega Alakazam: A --> A+: Disagree
It's fast, and powerful, but not powerful enough. It doesn't do enough damage to most things and is usually OHKO'd back. This is more from the experience of other people using Zam against me, but then again, maybe I just plan really well.
Smeargle: D --> Unranked: STRONGLY AGREE
Lost all niche it had when BP was banned and spore was nerfed. Really not too shabby in AG, but a terrible in OU.