I don't think mew is S rank material, Sure its unpredictable and can run alot of sets really well, But S rank is more served for pokemon known for doing a few things Really really well. Mew has that versitility yes, but its not the VERY best at one certain job.
It is the best nasty plotter, the best unpredictable threat in the tier, and a damn annoyance to even stand up to. For fucks sake I believe this thing got suspected quite a while back (yes the metagame was similar enough to today's meta to be comparable, except the ladder trends changing towards more of a snorlax meta which only make mew more of a threat to most ladder teams). I know suspecting isn't enough to make a pokemon S, but I'm dang sure this pokemon is up with the top 5 pokemon in UU along with Snorlax, Kingdra, Zapdos, and Victini, it can run a multitude of sets which are ALL effective in their own way, exploiting mew's simply amazing supprot and offensive movepool, everything is perfect for mew, it doesn't need support like something like SubCM Raikou (RaiKou hates hazards, especially toxic spikes) as it can deal with most hazards and can break through most walls with no problem or support needed
Alright, you want to see how devistating mew is? lets see it VS The Critically acclaimed "Best UU walls" (it actually has little to no checks and counters, but has exactly enough checks to keep it from beeing broken as one would say, except I personally think this thing a broken ass threat)
VS SPECIALLY DEFENSIVE SNORLAX
What Mew can do: +2 252 SpA Life Orb Mew Psyshock vs. 252 HP / 4 Def Snorlax: 399-472 (76.14 - 90.07%) -- guaranteed 2HKO
What Snorlax Can do: 0 Atk Snorlax Body Slam vs. 4 HP / 0 Def Mew: 100-118 (29.23 - 34.5%) -- guaranteed 3HKO after Stealth Rock
VS PHYSICALLY DEFENSIVE SNORLAX
What Mew can do: +2 252 SpA Life Orb Mew Aura Sphere vs. 252 HP / 4 SpD Snorlax: 390-460 (74.42 - 87.78%) -- 6.25% chance to OHKO after Stealth Rock
What Snorlax Can do: 0 Atk Snorlax Body Slam vs. 4 HP / 0 Def Mew: 100-118 (29.23 - 34.5%) -- guaranteed 3HKO after Stealth Rock
VS UMBREON
What Mew can do: +2 252 SpA Life Orb Mew Aura Sphere vs. 252 HP / 252+ SpD Umbreon: 255-302 (64.72 - 76.64%) -- guaranteed 2HKO
What Umbreon Can do: 0 Atk Umbreon Foul Play vs. 4 HP / 0 Def Mew: 186-218 (54.38 - 63.74%) -- guaranteed 2HKO (MEW IS FASTER SO MEW WILL WIN IF UMBRE SWITCHES IN)
VS PORY-2
What Mew can do: +2 252 SpA Life Orb Mew Aura Sphere vs. 252 HP / 252+ SpD Eviolite Porygon2: 213-252 (56.95 - 67.37%) -- guaranteed 2HKO
What Pory-2 Can do: 0 SpA Porygon2 Tri Attack vs. 4 HP / 0 SpD Mew: 91-108 (26.6 - 31.57%) -- guaranteed 4HKO
As you can clearly see, all mew needs to do is simply set up an NP and there is no wall in it's way to stop it, even without NP Umbreon is the only one who does not get set up on (If you really want to set up on Pory 2s and walls and the like, run NP/Roost LO 2 attacks, the NPRoost/Softboiled also helps deals with getting worn down by the hazards of HO and Stall Teams) There are very little special attackers that can boast this 1-turn set up power, (it's not supposed to hit right off the bat, even though it can hit hard right off the bat due to it's amazing SE movepool) Mew is by far one of the best Special Sweepers in the tier (Raikou and Zapdos are stronger off the bat, but Mew's set up movepool and offensive movepool put these 2 Pokes to shame, although Raikou and Zapdos are definatly amazing pokes and are worth using in UU) and
This is not even considering SubBulk Up And SD