How's this one going, Dramps? Are all the G Max Megas firmly decided and unchanging from now on? :D
As for Rapid Strike right now it's a tie between G-Luke and DrPumpkinz' submissions. First person to vote for either one or change their vote to give one the lead shall break the tie!Pokémon name:Mega Urshifu (Single Strike Style)
Mega Stone name: Urshifusite
Stats: 100 HP / 160 Attack (+30) / 115 Defense (+15) / 79 Special Attack (+16) / 70 Special Defense (+10) / 126 Speed (+29)
Ability: Nemesis (the user immediately uses Psych Up on entry)
New moves: Psych Up purely because the Ability would be flavorfully awkward without it - it doesn't need to run it!
The concept of Mega Urshifu (Single Strike Style) revolves entirely around its new Ability, Nemesis, which allows it to take advantage of common setup sweepers and countersweep if given a free switch, including if it comes in after an ally is knocked out. The goal is to make it an anti-setup Pokémon/countersweeper in line with Marshadow.
When determining its stats, I focused specifically on Pokémon that Urshifu needed to be able to threaten to fulfill its purpose ("pro-concept" targets), then zeroed in on the matchups that I thought would be the toughest without special attention: opposing setup sweepers with type advantages over Urshifu.
With this in mind, these stats are designed to secure favorable matchups with Landorus-T, Shift Gear Magearna, Marshadow (stealing back stolen boosts is too funny), Swords Dance Mega Blaziken, any Mega Lucario, Dragon Dance Mega Salamence and Dragon Dance Rayquaza after they've set up. The goal is that it can reliably force them out and punish them for setting up while still being healthy enough to take advantage of its newfound boosts. A straightforward set with Wicked Blow, Close Combat and Ice Punch takes care of all of these given maximum Speed, Attack and a Speed-boosting nature.
(It also manages to beat Pheromosa, another Pokémon in the Ubers metagame that can raise its stats, but barely - the +15 Defense is so it can avoid a one-hit KO from Low Kick reliably assuming no prior chip so it can hit back and one-hit KO, but this one is pretty shaky and leaves Urshifu vulnerable to priority, so it's unlikely to be a full countersweep. Even so, Pheromosa is a physical setup sweeper, and Urshifu's job is to inhibit physical setup sweepers - this is a relevant matchup all the same.)
Notably, if Urshifu hasn't already Mega Evolved by the time a setup sweeper comes in, the worst it can do is force it out prematurely; on the other hand, if Urshifu manages to Mega Evolve earlier in the battle while preserving its condition, simply coming in can put the opponent into a disadvantageous position, as it keeps its new boosts even if its target switches out without allowing itself to be defeated. For this reason, U-turn is an excellent choice for a fourth move - it allows Urshifu to Mega Evolve early on and then leave unscathed so that it remains a constant presence in the battle and the fear of its arrival can stop opponents from daring to set up.
That said, while that means it can easily tear through many disadvantageous type matchups under the right circumstances, I was careful to make sure that it retained several counters and wouldn't just wreck the tier. As it has no way to capitalize on Special Attack boosts and has abysmal special bulk for the tier, Urshifu struggles with its matchups against many common special attackers, including some that use setup moves, as well as some physical attackers that simply hit harder and faster without boosting. Flying and Fairy Arceus, Mega Lopunny, Mega Mewtwo X, Tapu Koko, Scarf Tapu Lele, Yveltal, Zacian-C and Scarf Zacian are all (probably viable) Pokémon that can reliably beat Urshifu.
On the less reliable side, Urshifu can beat Mega Diancie in neutral situations, but only if Diancie hasn't received a Defense boost from Diamond Storm, meaning Diancie can bypass any potential Nemesis shenanigans. Non-Mega Diancie, which has seen use in AG this Gen but may not be as relevant in a non-Dynamax meta, can reliably beat Urshifu without prior damage. Klefki of all things, though it prefers to run Play Rough, can also check Urshifu if it runs Dazzling Gleam.
Somewhat amusingly, considering their similarities, Urshifu also fears Ditto, which can copy its boosts, outspeed it and dispatch it with its own super effective STAB. One day, I want to witness a match involving a setup sweeper, a Marshadow, a Mega Urshifu and a Ditto.Actually, there's a much stronger option that fulfills a specific purpose, but I wasn't sure if it was worth the potential problems - the only intended benefit of this changed spread is that Urshifu can beat Zacian-C after rocks or any other chip. I would consider this matchup to be incredibly pro-concept - Zacian-C is one of the strongest Pokémon in the Ubers and AG metas alike, and it's one of the only Pokémon that always has a boost thanks to Intrepid Sword, so making it possible for Urshifu to take advantage of that would give it an immediate presence in the tier and take great advantage of its signature Nemesis.
This is the variation that can beat Zacian-C:
Stats: 100 HP / 161 Attack (+31) / 115 Defense (+15) / 65 Special Attack (+2) / 60 Special Defense / 149 Speed (+52)
(There is no way for Urshifu to beat Scarf Zacian with either stat spread.)
Unfortunately, due to its type, the only way for this to be possible is by outspeeding Zacian-C, which means a massive boost of 23 points to Speed (and 1 to Attack to remove a roll). This Speed increase most likely has unintended consequences on many other losing matchups that may or may not be desirable.
With that said, I'm somewhat worried that this stat spread is too good against everything else - in the sense that it makes Urshifu less dependent on acquiring boosts to be successful. While I obviously want it to be able to succeed in the Ubers meta, I also consider it important to keep the focus on Nemesis and not just make a generically strong Pokémon. Urshifu is meant to deter haphazard use of setup moves, but the goal wasn't really to make it an insanely good win condition even when the opponent doesn't give it any boosts to steal.
That said, I might be overestimating this spread, and it's possible that losing to such a prevalent Pokémon would do more harm to its place in the tier than having a few unintended winning matchups; base 160 Attack without the benefit of an item might also be on the low end for an offensive Pokémon in the tier, making its weak Special Defense more exploitable and still incentivizing it to steal boosts wherever possible.
With that in mind, this alternative is still a serious consideration, and I would appreciate feedback on which spread would be the better choice!