While I'm at it, I would like to also make a few nominations -- Kyurem-Black from A to A-, Blacephalon from A- to B+, and Mimikyu from B- to C+.
from A to A-
Honestly, Kyurem-Black is by no means a "bad" or even necessarily much worse Pokemon in the current metagame than it was before, but it clearly has lost a lot of popularity and ability to fit onto teams over the past month or two. While the increase in viable Defog users is admittedly beneficial for the Stealth Rock weak breaker, the previously dominant core of Tapu Koko + Kyurem-Black has taken a significant hit over time. I feel like some of the main reasons as to why it is falling a bit in popularity are competition and metagame trends. The former mostly is the fact that there are so many other solid Z move users out there, with SD Kartana being all over the place, Z Heatran rising quite a bit in popularity, and conventional picks like Tapu Koko, Greninja, Volcarona, Magearna, etc. also still being relatively popular with a Z crystal equipped. The LO set is, of course, still viable, but I feel it has always been a bit less potent as it loses the nuke factor, relies a bit more upon Terrain support, and can be walled by a few more Pokemon (or forced to recover quicker). As for metagame trends, the metagame has gotten a bit quicker on the offensive front whereas the bulkier side has integrated some decent counterplay to Kyurem-Black over time, in terms of consistent hazard play (i.e: rocks almost always being up), more Tyranitar, Clefable and Heatran to situationally check it, and Toxapex running Toxic more often to directly whittle Kyurem-Black. Overall, Kyurem-Black is still solid, imo, but it has taken a bit of a hit and putting it in a sub-tier alongside prominent Pokemon such as Clefable and Tapu Bulu or devastating win conditions such as Volcarona simply does not add up -- it fits more in a solid, but not outstanding or remotely metagame defining sub-tier such as A- with dudes like Mega Pinsir, imo.
from A- to B+
The clown hype is absolutely absurd in this thread, to be honest. I really cannot believe that we had to vote for it to potentially go up to A because some people in this thread were so hellbent on it actually being good. In my opinion, Blacephalon is pretty hit-or-miss and the opportunity cost of using it as opposed to something that is more consistent with an actual defensive presence is pretty huge in a metagame with Tyranitar and Toxapex practically on every other team. People like to say "oh, it's so good in the other match-ups" to the extent that it makes up for it and I mean yea it is effective, but there is still decent checks to it here and there, most of them offensive in nature. I feel like the new toy factor behind it is fully warn-off and the true colors of Blacephalon as a fairly mediocre addition to the metagame that will be good enough to hang in OU by usage, but not all too much better are starting to set in. I do not consider Blacephalon remotely top tier to the extent that it can fit into A like many propose and I think even A- is a stretch just because it sorta is defined by a weird, niche role as opposed to a consistent spot on teams due to how hit-or-miss it really is, as I said before. Like shit man, if +0 Adamant Mimikyu can kill you from full with Shadow Sneak 90% of the time and you're not even 4x weak to it and you want to be taken seriously in this tier, you have to have an insane and consistent upside offensively -- like Kartana does to make up for horrendous special bulk -- and it simply does not have it all there for the aforementioned reasons. Overall, Blacephalon is just a really average Pokemon with a lot of downside that so many posters in this thread either understated or completely ignored and I think it stands out in A- as one of the most inconsistent Pokemon in that sub-tier and I feel that B+ fits it better overall.
from B- to
honestly unrank this bitch C+
Mimikyu is awful -- this shit's too weak and it has a godsend ability yet it fails to cash in far more often than not. It is an excuse for a check-all that gets slapped onto poorly constructed teams or cheese in order to manipulate certain scenarios and really should not be used on any serious teams. Therefore, I would like it to not be remotely associated with the B rank/any of its sub-ranks! I could elaborate more, but I feel like enough people will get why Mimikyu is just bad and should never really have been this high.