Naw I think Keldeo can go to A-. It gets walked on by a lot more things in the tier just by virtue of its typing than last gen when its typing actually was a strong point for it (tapu bulu and toxapex immediately come to mind) and as the poster above me said the drop of usage of certain pokemon hindered the the viability of what was its best set at the beginning of the gen which was scarf. Tapu lele and tapu koko both take giant turds on it regardless of the set and latios and mega pinsir both outspeed and embarrass it lmao... do keep in mind that these are all A rank pokemon that I am mentioning which should give the notion that these are prevalent enough mons for using keldeo to be a bit of a liability. I also think the sets that have any sort of move that require setting up are just given a tough wrap because it finds too little opportunities to set up because almost every team has at least one natural check to it. Also now that I think of it mega venu is just on the rise and that's just a hard counter in every single way. In fact I've just realized how much I hate using Keldeo in this generation... I would personally put it as far as B+ but my personal viability rankings would be much more distorted than the one currently up... it simply just got worse at the beginning of the gen when all these new good mons that were hard counters/checks to it came out but now the rise of mega venu and the drop of all these pokemon that is handled with its best set are just examples of adding salt to the wound. Granted Keldeo is a great mon most of the time but it just seems there are all these glaring issues that used to not be so noticeable and that's why I think it should drop but that's just me...
Did you say that Mega Pinsir outspeeds Keldeo?
I encourage you to stop thinking that Toxapex and Bulu are counters when they only counter the Sets you used. Taunt completely shuts down Pex and Bulu dies to +1 Hydro Vortex after some prior damage. 98.2% is the max roll.
While Keldeo is very easy to check with the Tapus it is very hard to counter aside from Mega Venu and Latis, both which are food for CB TTar. Nothing can really safely switch in and you do not have to CM. There are matchups when the good defensive typing with its solid bulk are more appreciated than the ability to sweep. Z-Hydro is just a nice nukebutton that simply kills of Lele, Mega Pinsir and other fairly bulky mons. I cannot describe how many times I have switched Keldeo into Specs Gren, Secret Sworded while getting a kill and my opponent trying to force me out with their Specs Lele, just to die from neutral Hydro Vortex or Mega Pinsirs trying to setup, thinking I am locked into Secret Sword.
You said Keldeo has a hard time setting up but that is not true either. Having perfect counters to the Scarfsets such as Bulu, Toxapex, Tangrowth and even Mantine makes the opponents play very obvious which gives you a free CM. Mega Venu also only counters if it has Giga Drain, which it occasionally lacks.
I do not really mind if Keldeo drops in the end but I just cannot stand it when people only consider Scarf Sets and think all you need is a Toxapex or Mantine to be safe.
Regarding Mega Altaria:
I want to share my results with the DD+Cotton Guard Set with Facade and Roost. People mention Celesteela and Mega Venu as counters which is true,
if you Mega Evolve. While you can beat Celesteela even if you Mega Evolve with all the defense you get, you risk getting crit. I personally prefer the Set that barely outspeeds Gengar at +1, max HP and rest into attack with Adamant nature. What is the result? I never really swept with that Set. Against Balance all your opponent has to do is Leech Seed with Celesteela, go to Pex and Haze all your boosts. Against HO you get pressured too much without Earthquake and against Stall you cannot break Unaware Clef+Toxapex. Overall not a reliable Set to sweep with. Not enough experience with other Sets but I think it can drop to C+.
Regarding Mega Latias:
I did not use this mon yet but I faced many opponents using CM Stored Power with Tbolt, Bolt Beam Reflect Type, Bulky Roar and even Defog Sets. The most annoying Set I have faced was an offensive hazard stacking team with Greninja who use Mega Latias to Roar and threaten Defoggers like Zapdos, Skarmory, Mantine and Fini with Bolt Beam. Very disgusting Set because it is soo bulky and always Roars you into your hazardweak mons lol.
CM Stored Power with Thunderbolt is a wonderful wincondition that can setup on so much in the meta, even against other CMers and beat them with Stored Power. Thunderbolt hits relevant Darktypes hard except for TTar and gives you a tool to hit Pex+Steela hard with few boosts.
I could see Mega Latias rise to
A- easily. Its defensive typing is so good on almost every team. I might use this today because my teams are always weak to Zapdos. Discharge though :/
Regarding Mega Diancie:
I basicly used every Set you can think off and it is incredible how versatile Mega Diancie is with the Speedbuff. Not having to run Protect allows you to run Sharpen, CM and even Rock Polish Sets much more reliably. You can now also setup in base form for the bulk and only Mega Evolve, when you need the immediate Speed or damageboost. Stealth Rock Sets are also very cool but you have to run HP Fire to punish Mega Scizor. Acting like you do not have the move early game is recommended to reliable lure Mega Scizor later and keep up your Rocks.
Also a fantastic Mega Pinsir counter and in general solid against HO and Stall with the correct Set. With Rock Polish (you have to Rock Polish as you force out Pelipper etc.) you can also 6-0 Rain teams if you weaken Ferro and Swampert.
I could see this mon going A+ in the future but because of Dugtrio and how every time you have to hope you picked the correct moves for the matchup,
A is fine. Stealth Rock, Stabs with HP Fire is probably the most consistent Set.