C+ ---> B- / B
Mew has so many good sets such as suicide lead, cosmic power, and even the transform/block/imprison set on stall teams (I once got beat by a random dragon dance weakness policy set too) that I just don't think it fits in with more niche things like Sylveon and Diggersby because Mew is often the best at what it does in addition to being versatile and unpredictable. I've often used it as a suicide lead for hyper offense because it has a movepool (stealth rock, spikes, taunt, explosion, flare blitz, tailwind), sits at a good speed tier, and has the bulk to forego focus sash in favor of mental herb to beat terrakion/grimmsnarl leads (red card is also an option). I can't speak as much for the other sets as I haven't used them myself, but from what I've seen I just don't think Mew belongs with the stuff in C+, a subrank below Xatu, and two whole sub ranks below Tyranitar.
I'd go further and nominate Mew to
B+. I think I nominated a while ago back when Home first brought over the new stuff including what I consider to be a significant buff to Mew with recovery moves. Yet in both updates to the rankings since, it hasn't moved places and hasn't really been discussed despite numerous other nominations for it.
I think part of Mew's current ranking is due to past gen biases that Cloyster similarly faced earlier, where it's seen as too gimmicky or inconsistent because that's how it was last gen when there were better mons to counter it. But in the current metagame, Mew's 100 across the board stats plus giga movepool is very good.
Usage isn't the best stat to bring up because it can be misleading in some cases but I think it's relevant to point out that the top 19 in usage are all A- or above (with Bisharp being the only A- which I think is wrong anyway) then 20th is Cinderace who is a starter, fun to play with, still B rank and some people have been nomming for B+ as well, and then 22 is Mew, who went up 2% in usage. The only comparable mons who are used much more than their current rank are Pelipper and Alolan Ninetales which are required for certain playstyles.
Mew is incredibly splashable as a custom fit to round out teams while simultaneously having sets that function well in many teams. Milkyway above has already pointed out some of them, I'd like to point out this set I've been running continuously at the top of the ladder basically since it came out.
Mew @ Leftovers
Ability: Synchronize
Shiny: Yes
Happiness: 160
EVs: 252 HP / 252 Def / 4 SpA
Bold Nature
IVs: 0 Atk
- Calm Mind
- Stored Power
- Soft-Boiled
- Aura Sphere
Yes it's similar to Cosmic Power, I argue that it's better than Cosmic Power. The ability to OHKO most mons at +2 with stored power and 2HKO walls is really good.
This is Cosmic Power
4 SpA Mew Stored Power (100 BP) vs. 0 HP / 4 SpD Dragapult: 136-162 (42.9 - 51.1%) -- 3.1% chance to 2HKO
Vs Calm Mind
+2 +2 4 SpA Mew Stored Power (100 BP) vs. 0 HP / 4 SpD Dragapult: 271-321 (85.4 - 101.2%) -- 6.3% chance to OHKO
Above that just speaks more to CM
4 SpA Mew Stored Power (140 BP) vs. 0 HP / 4 SpD Dragapult: 192-226 (60.5 - 71.2%) -- guaranteed 2HKO
+3 4 SpA Mew Stored Power (140 BP) vs. 0 HP / 4 SpD Dragapult: 475-561 (149.8 - 176.9%) -- guaranteed OHKO
People running cosmic power so they can epicly stay in on Dracovish in the rain need to wake up a bit.
This set with some support (I think cleric support is most important, screens is good too) is super strong and could be improved possibly with better support I haven't thought of because I don't play balance teams much. It still outspeeds all walls, survives stupid things like life orb Bisharp Knock Off, outstalls Mandibuzz without Toxic or Taunt, kills Unawares through Stored Power, can eat Timid Specs Drag Shadow Ball at +2 so if you don't get the drop (you still have a chance after recovering) you beat it, etc.
There are so many sets that aren't even being attempted like choice scarf trick, ice beam/nasty plot/sub/roost, double dance NP/agility/stored power/aura sphere, iron defense/body press/roost/stealth rock, etc.
The arguments against Mew are often "jack-of-all-trades master of none" but it basically is the master at some of these sets as far as I can tell, I think it can do the same thing Necrozma does but better due to speed generally. And there isn't a single pokemon in the entire game that can come in on it 100% safely. Every time you make a play into Mandibuzz you gotta hope it's not Nasty Plot ice beam, every time you go into Hippowdon you gotta hope it doesn't have taunt/ isn't nasty plot ice beam lol. Every time you go into specs Dragapult you gotta hope it's not DD play rough. Or outright gonna play rough, or thunder wave, or w/e.
You get the point, I just don't see how it is worse than honestly any of the mons in B+ or below.