Mew staying UU wasn't much of a surprise. It's definitely a great, unpredictable Pokemon, but it can be revenged easily regardless of the set it's running and has some Speed issues.
Nasty Plot Mew is one of the best Special attackers in the tier because of its insane movepool, but 100 Special Attack scares no one when you're spamming 90-95 power moves. Plus, almost every revenge killer in the game can stop this Mew since UU's revenge killers often use Sucker Punch or Megahorn as attacks. Nasty Plot Mew is nice in that with a little damage it can destroy defensive teams, but it struggles against offense, gets revenged easily, etc.
Swords Dance Mew is arguably the best set in my opinion. SD Mew gets priority, bulk, a way to boost its stats, and decent Speed for a Physical sweeper. Unlike NP Mew, which commonly carries Leftovers to offset the lack of reliable recovery, SD Mew can step it up a notch with Life Orb. Unfortunately, that power boost is pretty negligible when your two reliable attacks are Zen Headbutt and a non-STAB Drain Punch. Sucker Punch is what makes SD Mew the best set in my opinion. If you can wear opponents down, SD Mew becomes an obnoxiously good cleaner. Heracross, Honchkrow, and Hitmontop may be able to defeat a Mew normally, but if you can wear them down to 30, 40% or so, then Mew isn't afraid of them anymore. Drain Punch's recovery is also great because if an opponent's Pokemon is at 40% or so, chances are that a Drain Punch will KO them at +2. If that's true, then you can forego the shaky Zen Headbutt and reclaim 20% of you health with a Drain Punch.
Baton Pass Mew is exactly what it says on the tin: a more versatile, less threatening Venomoth. Venomoth is by far the best Baton Passer in the tier because it can pass Quiver Dances. Meanwhile, Baton Pass Mew needs multiple turns to pass (Special) Attack boosts and Speed Boosts, giving the opponent two turns to react to the Mew user's strategy. On the other hand, Venomoth just QD's up for a turn, passes to Togekiss, and watches as Togekiss cleans up even the most prepared teams. Baton Pass Mew is nice because it can use two attack moves while Venomoth can only use one, but Venomoth can put people to sleep and choose one of two gruesome abilities to facilitate its passing.
If Mew was packing 110 or 120 Attack/SpA stats with that movepool, it'd be in the BL tier right now. However, Mew just isn't tough enough to bring UU to its knees.
Speaking of Venomoth, by God is it a good Pokemon. Venomoth+Togekiss is the most lethal BP chain in the game right now (eat your heart out, Nidoking). Togekiss' great coverage with Aura Sphere+Air Slash makes it an utter beast at +1/+1. Add in the Special Defense boost and it's practically invulnerable to Special attacks. Venomoth's Wonder Skin makes even teams prepared to counter it fail half the time, while Tinted Lens lets Venomoth flex its muscles at those who want to kill it before it can pass.
how is it still bl2 smh...