Mega Altaria S -> A+ Neutral, but inclined to agree slightly. Comparatively I'd say Altaria has currently slumped below Char X, is comparable to Metagross and Gyarados, but above Diancie (who I think should drop to A personally) in effectiveness. While its typing is great, it isn't initially, and its SR weak. Coupled by its speed, it can be hard for the sweeper set to shine. Metagross comparatively gets an immense speed boost, and Gyarados, while SR weak, does as least have Intimidate to prompt switches. It is indeed versatile as all hell, but I think it has reached that level of jack of all trades but master of none, that I don't think defines S rank currently.
Mega Gyarados A+ -> A Disagree. I can see why this nomination is here, but I don't agree with it. Intimidate is more useful in a more offensively inclined meta actually providing Gyara with more set up opportunities, and is a DD sweeper that need not wait until late game to attempt a sweep. Gyara's bulk and typing makes it an amazing Healing Wish recipient and perhaps the mega most likely to set up a second time after being switched out. Something that is often overlooked is Gyara's typing makes it resistant to a whopping 5 types of priority, meaning that if their team has their scarfer eliminated they will be hard pressed to knock out Gyara, especially when CB Talonflame, Dragonite, Breloom and Conkeldurr users aren't exactly en vogue currently. Finally, DD + 3 attacks is a thing, and you need not use both STABs. Either Crunch or Waterfall can function as amazing mono-STABs thanks to good neutral coverage and their secondary effects and Gyara can be a far more threatening or potent sweeper if you decide to run 2 coverage moves like Iron Head for Altaria and Clefable, Ice Fang for Venusaur, Garchomp and other 4x weak mons, or EQ for Manectric, etc.
Mega Alakazam A -> A+ Agree. Been trying out Zam lately and man, while we all know how fast 150 is on paper, its something else entirely when you see the opponent's team and how self assured you feel knowing you have something this fast and powerful on your side without requiring a boost. It really isn't a sacrifice of a mega at ALL. Pick any standard semo-stall hazard stacking core to whittle the opponent's team and barring things like Chansey, Zam can finish just about anything it wants with its great neutral coverage, or revenge damn near anything offensive it wants to as well. Its so fast it can revenge +1 Gyarados (barring Focus Miss). And while its frail, Trace really helps alleviate this to a degree like Heatran, Manectric, etc. Then of course there the fact that it outright counters almost every weather dependent mon thanks to Trace. Having all of that in one package is REALLY not sacrificing your mega slot, certainly not for climbing the ladder. The main downsides are of course forced out by any and all priority (and even then, Reflect on a predicted switch can mitigate this to a degree), horrible mono-Psychic typing for STAB and defensive purposes, reliance on Focus Blast, and a degree of 4MSS. You really want coverage moves like Shadow Ball, Energy Ball, Dazzling Gleam and HP Fire to take out would be walls, but there's also lots of uses in Encore, Taunt, Reflect, and even Calm Mind end game. Regardless, he has proven to be an amazing asset when I use him, especially for smart players who can make the most of Trace.
Starmie A -> A- Disagree. Its speed tier is too useful for it to drop, and it has plenty of tools to be a continued asset, albeit it can certainly be spread too thin. It's still the best offensive spinner there is barring Excadrill in sand, and has great matchups against most hazard setters with Analytic Hydro Pump and Ice Beam, and its still one of the best switch ins to Scald thanks to its typing and Natural Cure, and even handles Sub-CM Keldeo with Recover and Psyshock alone. Of course it can't do everything at once, but between its speed tier, utility options, and good offensive presence if need be, I think it remains a cut above A-.
Smeargle D -> Unranked A-effing-men to that.