Mew @ Leftovers
Ability: Synchronize
EVs: 252 HP / 224 Def / 32 Spe OR 176 HP / 252 Def / 80 Spe
Bold Nature
- Taunt
- Will-O-Wisp
- Seismic Toss
- Roost
Physically defensive Mew was something considered in the XY metagame, but it was never really run because there was so much value in SpD in the metagame including checking the likes of Greninja/Thundurus. Mew also ran a lot of Speed at that time, which isn't really conducive to the defensive role that physically defensive Mew likes to play in this metagame. Physically defensive Mew was never a standard choice, but it has a lot of merit in this metagame because it handles some top threats with high effectiveness. Physically defensive Mew counters Mega Metagross (bar Attack raises), Mega Lopunny, Azumarill, non-Life Orb Excadrill, Mamoswine, and non-SD Garchomp off of sheer defensive might while not caring one bit about Scarftar, being able to beat a variety of physical attackers in 1v1 scenarios thanks to Will-O-Wisp, checking Diggersby with the faster spread, and having the ability to Roost off Mega Beedrill's U-turn with relative ease if it is at high health. By the way, Seismic Toss is run to do more consistent and significant damage to some of these Pokemon so that it can actually deal with them in a quick/efficient manner; it is also for breaking Mega Lopunny's Substitutes.
Here are some calcs that help illustrate some of the things I mentioned:
252 Atk Tough Claws Mega Metagross Meteor Mash vs. 252 HP / 224+ Def Mew: 157-186 (38.8 - 46%) -- guaranteed 3HKO after Stealth Rock and Leftovers recovery
252 Atk Mega Lopunny Return vs. 252 HP / 224+ Def Mew: 127-151 (31.4 - 37.3%) -- 83.4% chance to 3HKO after Stealth Rock and Leftovers recovery
252+ Atk Choice Band Huge Power Azumarill Play Rough vs. 252 HP / 224+ Def Mew: 198-234 (49 - 57.9%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
+6 252+ Atk Huge Power Azumarill Aqua Jet vs. 252 HP / 224+ Def Mew: 234-276 (57.9 - 68.3%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery (it dies to burned +6 Knock Off, but it can cripple Azumarill with a burn which makes it much easier to deal with)
252+ Atk Excadrill Earthquake vs. 252 HP / 224+ Def Mew: 135-160 (33.4 - 39.6%) -- guaranteed 3HKO after Stealth Rock, sandstorm damage, and Leftovers recovery
252+ Atk Life Orb Huge Power Diggersby Return vs. 176 HP / 252+ Def Mew: 203-239 (52.7 - 62%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
252 Atk Adaptability Mega Beedrill U-turn vs. 176 HP / 252+ Def Mew: 248-292 (64.4 - 75.8%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery (so assume 70% roll then it's only taken 14% net damage if it goes for Roost)
252 Atk Tyranitar Crunch vs. 252 HP / 224+ Def Mew: 198-234 (49 - 57.9%) -- guaranteed 2HKO after Stealth Rock, sandstorm damage, and Leftovers recovery
The biggest advantage Mew possesses over, say, Slowbro, is Speed. Mew is almost as bulky as Slowbro overall, and while it obviously doesn't beat everything that Slowbro does, its Speed and breaking capabilities with Taunt + Wisp give it an advantage. Mew also beats Grass Knot Mega Metagross, which Slowbro does not, and cares less about Toxic Mega Lopunny wearing it down because Synchronize ensures that Mega Lopunny cripples itself in the process. Beating two of the best Megas in the tier is more than enough for physically defensive Mew to have a niche. While it's often stated that Mew is useless against Mega Sableye, this couldn't be further from the truth; Synchronize allows Mew to burn Mega Sableye, which can be absolutely crucial in wearing it down and beating it over the course of a match. Mew also still has access to its trademark Taunt + Wisp combo, which still allows it to take advantage of a variety of defensive threats effectively. Finally, I feel like physically defensive Mew is better in the metagame as a whole than offensive lure versions of Mew, which are incredibly team-specific and squander Mew's excellent defensive capabilities.