To B-
Kingdra fits on only 1 team archetype: rain and it functions there as a sweeper/ wallbreaker. However, it struggles to break/ sweep consistently. First, balance teams now heavily utilise fully SpD Toxapex, which can eat any hit from Kingdra then recover or pivot accordingly. Second, offensive teams with Chansey are on the rise and the pink blob eats hit even better than Pex. Even without the blob, offense still has counterplays like Magearna, Mega Alakazam, Mimikyu, Greninja, priority or just sack TTar to bring sand then take advantage of a weak and slow Kingdra to set up. Finally, against stall, while uncommon, Kingdra has no realistic chance to break stuff there.
Another thing that warrants a drop for Kingdra is this: rain teams have outgrown Kingdra. It is no longer THE mon to use on rain like it once was. Good, consistent rain teams can be built without Kingdra rather easily. Usage stats show this too since Kingdra's usage is significantly lower than Pelipper's and Swampert's.
Kingdra fits better in B- with Quagsire, Skarmory (niche stall mon) or Stakataka (niche TR mon) or Victini (niche on ONE outdated team) or Mimikyu (niche on ONE outdated playstyle) or Latias (bad
niche scarfer on offense)
to C+ (or B-)
This thing's niche is countering Kingdra and SG Magearna so when both threats become uncommon and less viable, it should definitely drop. I explained Kingdra's fell above and as for SG Magearna, it's simply uncommon atm to warrant a teamslot to Gastrodon.
Some one-liners cause most things have already been said or there's just not much to say:
Mega-Aerodactyl to UR: there's absolutely no reason to use this. It's weak and frail and there are better Flying checks
Lycanroc-Dusk to UR: use Terrakion
Tapu Koko to S- : you cant build an OU team with an actual Electric resist because of this thing. It is extremely influential in the tier and more than deserves to be S-
Tornadus-T stays B: it is not that good people. Don't get carried away with the hype. Also, with Koko going to S- getting a lot of support, it makes little sense for this to rise at the same time.