haven't really used
mega blastoise much these days, but the times i did use it it was very underwhelming in what it does. as a hazard remover it's got better things to be doing than spinning and as a check to things like entei, you'll never get enough mileage out of it to entertain a serious build around it. from what i can interpret there are just better water-types to run sometimes on a team, like suicune or slowking if you're looking for a check to entei and gyarados or feraligatr if you're looking for something particularly destructive that can actually do something to p2 or florges. a water-type spinner if you're being specific? tentacruel works.
tl:dr it's middle ground between a bulky water and offensive water, but doesn't really excel in either aspect.
a- seems fine as it's still one of the better megas
since there hasn't been much discussion with the lack of nominations I've got a few points we could all touch on if that's alright
A to A-: With the return of Sableye and the prevalence of other bulky-psychics, especially Slowking who both bring a ton more defensive utility and splashability as it offers itself as a beneficial, more like necessary, water-type that all teams need, so Reuniclus is in a very tough spot in the current metagame. Slowking also preforms incredibly similar roles as Reuniclus whether it be OTR which is really hard to justify using in today's metagame and the defensive Calm Mind set.
B- to B: With spikes stacking teams becoming very common, Qwilfish fits as a nice setter that also works as a beneficial fire, fairy, and fighting-type check that can fill in several gaps at once that a spikes stacking team may have. It's also got nice utility lying in it's movepool with access to Thunder Wave, Taunt, Scald, and Pain Split for whittling down high HP mons like Blissey or Suicune.
B- to B: Aerodactyl is starting to receive the recognition it deserves as possibly the best suicide Stealth Rock setter in the tier as it's very reliable and can actually function beyond that of a suicide lead as it can get multiple turns to reset Stealth Rock.
C- to C: Accelgor fits in a similar vein with Qwilfish. The rising popularity of spike stacking often makes it a pretty valuable addition towards a team, but in this case more offensive builds. It's also got insane speed, Encore, and can "sacrifice" itself with Final Gambit. It's worth noting it can run a pretty fine offensive set as well with it's okay movepool making it a good revenge killer as well.