Gonna piggyback on Zygarde to A+ (was planning to make this nomination myself lmao)
Zygarde is one of the most consistent Pokemon in the tier, definitely as consistent as an A+ mon, and it's just as versatile as one too. Choice Band hits hard as hell, SubCoilToxic makes Stall fall over, Dragon Dance allows it to out pace the a lot of threats with its good 95 Speed, it can run Extreme Speed for priority or Outrage for a Dragon nuke, and it can even make use of Groundium Z. On top of that, Zygarde's typing lets it serve as a reliable answer to the threat of Volcarona (bar HP Ice of course), and to Zard Y as well. Thousand Arrows is the only coverage Zygarde needs, so its other 3 move slots are essentially free, and Zygarde can be tailored to fit what your team needs. If a team is unprepared for Zygarde, it will eventually be swept by Zygarde. It is a threat that definitely needs to be taken into account when teambuilding. Definitely worth of A+.
On the flip side, also gonna piggyback on Zydog to B-/C+
I don't think Zydog is as bad or as niche as the mons in C+ like Hippowdown or Whimsicott, due to Thousand Arrows, its great speed tier, and offensive power equal to Zygarde, but it faces so much competition from Zygarde that it definitely needs to drop. Its pitiful bulk also make it susceptible to priority, and it providing minimal defensive capabilities to the team. There isn't really much else to say about Zydog other than just use Zygarde instead.
Keep Tapu Bulu A-
Tapu Bulu may not be the best Tapu, but it's by no means a B+ mon. The support it provides with Grassy Terrain is quite useful, and it also makes Majin Bulu Tapu Bulu hit like a truck. Anything not named Celesteela, Skarmory, Mega Scizor, or Mega Metagross has a difficult time switching in to this mon. Its SD Fightinium Z set is very difficult for Stall to handle too. Tapu Bulu's typing also allows it to be arguably the best Zygarde answer in the tier, and it can handle non Ice Beam variants of Ash Greninja, Keldeo, Lando-T, Dugtrio, and other mons. Dropping it to B+ would be a mistake, as pointed out earlier. Bulu isn't the strongest mon in the game, but it should never be slept on.
Drop Garchomp to A
I've never had any issues with fighting against Garchomp. When using it it hasn't really impressed me, or disappointed me. It's about what I expected. It's a really solid mon, but not as solid a the rest of A+. Its Scarf set is useful in that it outspeeds Salamence and Volcarona at +1, but its STAB attacks have two very common types immune to them making it awkward to being locked in. On top of that, its coverage moves just aren't that strong without SD due to their low base power and lack of STAB. It has a lot of issues with fighting top tier mons like Lando-T, Celesteela, and Tangrowth. I feel like it's being oversold as a mon in A+, and it looks more at home in A.
Not gonna comment on Keldeo. I haven't used it enough to give a solid opinion on it, and I've rarely faced it, though its performance in SPL is impressive.
I don't think I have anything to contribute with Greninja (Stay in S) and Mega Scizor (Rise to A), so I won't give them a paragraph.
Also would like to bring up Volcarona to A
Volcarona may be one of the scariest Pokemon to face in the tier. A team weak to Volcarona will be swept by Volcarona. Of course, Volcarona requires support as its Stealth Rock weakness is pretty bad, but its pros far out weight its cons. Fire (With Firium Z) + Grass coverage is stupidly hard to switch in to (Only like Zygarde, Garchomp, Heatran, Toxapex, Chansey and Mantine can do it), and Volcarona can run HP Ice or Ground to eliminate 2 of its 6 answers. It can even run Bug Buzz if it fears Latios for some reason. It definitely must be in mind when teambuilding, as it basically defined a good scarfer in this meta as something that can outspeed it at +1 and kill it. Volcarona also has a good match up vs Pheromosa, and it can punish U-Turners with Flame Body. Volcarona just seems better than a lot of the mons in A- to me. I don't really have Jirachi, Keldeo, or Mega Alakazam in mind when teambuilding, but I definitely have Volc in mind. It should rise to A.