OK, as this thread died, I'll just post some brief opinions on the slate.
Mega Altaria S -> A+: Personally, I'm completely neutral on this. Altaria is a dominant force with debatably the most incentive for use out of any mega, and it can be slapped onto almost any team without any real drawbacks. However, it does have Garchomp syndrome in that respect, so it is questionable as to whether that is enough to keep it in S.
Mega Gyarados A+ -> A: This thing is a monster. It is easily one of the most threatening DDers in the meta atm; personally, I think it is definitely a step above anything which is in A atm, and I honestly don't think it should move down
Heatran A+ -> A: This thing is so freaking splashable; seriously, I see this thing one one in every two or three teams; usage isn't a good argument, but the reason for that usage is: Heatran can easily fill a role on pretty much any team in the meta due to its stellar typing (immune to all /common/ passive damage aside from leech seed (if you want techicalities, it is also hurt by hail, Curse and all variants of Wrap barring Fire Spin and Magma Storm), stellar movepool (access to great utility options in SR, Taunt, Roar and Magma Storm (although imo the latter isn't that good in the current meta), great offensive moves to take advantage of including Fire Blast, Earth Power, Flash Cannon, Stone Edge/Ancient Power, Hidden Power and Power Herb Solar Beam) and variety of viable sets it can run, including ones which let it beat some of its common checks (Air Balloon allows you to switch in on grounds and beat them with the respective coverage move (fire blast for exca, HP Ice for any others barring Hippo - which you are better off not coming in on unless you know its full PhysDef), Power Herb allows you to beat Azumarill 1v1 with Solar Beam), which allows it to be greatly customised towards the team's needs. I'd compare this thing to Garchomp and M-Alt all day when it comes to splashability, and I can't see it ever dropping personally.
Mega Alakazam A -> A+: I've not used or seen this thing much, so I won't comment.
Weavile A -> A+: Completely neutral on this. Its dominant in the meta atm, but the fact that it falls flat v.s. bulkier teams may hold it back (although a number of people feel its time for the move, so idc what happens with this).
Starmie A -> A-: I have a lean towards no based on the fact that its offensive set can currently rip portions of the metagame apart completely due to its great speed tier resulting in it forcing so much out to eat an analytic-boosted hit, only for it to be able to outpace most things on the following turn anyway. A common switch-in for this is AV Torny-T, but even that takes a minimum of 62.2% from an analytic-boosted Ice Beam, leaving it at a big HP defecit when it switches out again. I understand that the rise of things like Weavile and Manectric hurts it, but I personally think it is a step above things like Serp and Rachi in the meta's current state.
Smeargle D -> Unranked: This shitmon can go and die in a hole for the cancer it caused before the new BP clause was implemented. Good riddance is all I have to say.