But the question I want answered is if whether or not would be Mew better than Jirachi in OU? Would Mew usurp Jirachi's S1 rank here:
https://www.smogon.com/forums/threads/dpp-ou-viability-ranking-mk-v.3683332/ ?
(if Mew can't take Jira's #1 spot, then Mew is OU right?)
Edit: I also just realized that Mew is UU in the next Gen (BW) as well
Pros Jirachi has over Mew:
Better Typing (notably not weak to Pursuit/U-turn, resists Bullet Punch/ESpeed/Dragon/SR, immune to Poison)
Better Ability (Serene Grace is highly abusable, Synchronize not so much)
Pros Mew has over Jirachi:
Larger Movepool (though Jirachi's is vast as well)
Notably access to stronger stat boosting moves like SD and NP as well as Explosion/ASphere/Softboiled/Wisp/Taunt
Best Baton Passer in the game
Both are able to run versatile sets
Both have exact stat distributions
Honestly tough to say if Mew would be considered to be the 'very best' in OU over Jirachi. The quality they both share is that both are "uncounterable" at first glance and require the player to know what type of set it is running.
Thanks to the combination of WoW , Soft boiled, and its natueal bulk, Mew would be a pain to deal with, and a really unhealthy adding to the DPP metagame.
Imo this could be one of Mew's best sets:
Edit: I just noticed a very similar set is on Mew's BW page (at least my line of thought was correct then)
Mew @ Leftovers / Colbur Berry / Kasib Berry
Ability: Synchronize
EVs: Unsure
Unsure Nature
- Taunt
- Will-O-Wisp
- Softboiled
- Seismic Toss / Psychic / Surf / Earthquake / Thunder Wave
Would function similarly to the Gliscor stall-breaker set imo.
Answers to this set might be Gengar/Rotom/Jirachi/Trick/Toxic Spikes/Paralysis*/Faster Taunters
Scarf Scizor could also blast it with U-turn**. Band Scizor could play mind games with BP/Pursuit on a weakened Mew.
Scarf/Lum Ttar could mess with it as well (Specifically Lum Ttar could DD on the Wisp or just play with Crunch/Pursuit).
Scarf/Specs Heatran could intercept a Wisp**. Or just wall the set depending on Mew's moveset.
Also maybe Lum Machamp or set-up sweepers holding a Lum Berry like Gyarados and Dragonite.
WoWs faulty accuracy could also be factored into this somehow.
* I want to mention that Syncrhonize has a niche use in foiling paralysis plans, for example JIrachi Para+Flinch
** U-Turn to Heatran on the Wisp?? Sounds like a good combo
Even if I don't think it would be as good as a defensive set, an offensive set with NP+ 3 attacks would be a threat too, especially with Mew's movepool.
I want to mention that Azelf is a similar pure Psychic in OU with access to NP and good coverage moves (as well as Explosion) with higher attacking stats and speed yet the NP set is hardly considered too powerful for the tier (Azelf doesn't have Aura Sphere but can substitute with HP Fighting which is only 20 less base power).
You already answered yourself, but I think Swords Dance + explosion would be what drives everyone insane, in addition to all its other sets that probably can't be checked reliably.
Also outside of Explosion for the SD set for Mew, its other attacking moves are meh at best (Psychic-STAB, no good priority move). So the strat would be to predict the Explosion with a Ghost or Protect. Lucario/Scizor/Nape/Weavile/Kabutops are better OU SD sweepers I think.
Now that I'm thinking of it, a strategy would be to SD with Mew, U-Turn on the next turn to see the opponent's response. If it's a Ghost, switch to Scarf Ttar to Pursuit trap it. Then SD with Mew again and Explode on the next Pokemon. Then have another SD sweeper like one of the others I mentioned finish the team off.