Don't know I how I missed this one til now, was inspired by Shell Smash Slurpuff in Alphabet Cup, but Slurpuff + Unburden + Power Herb + Geomancy. Modest nature hits an absurd Speed stat of 972 after one Geomancy, meaning there is literally nothing in the game that can outrun it without their own crazy setup or priority. Even Timid Scarf Deoxys-S only hits 756. Priority is very common in this meta however and Slurpuff is Bullet Punch weak, and it's power may be somewhat underwhelming compared to many of the other setup sweepers in the meta (can't beat the blobs for example). Also has some 4MSS, as it needs Geomancy and a Fairy STAB (Dazzling Gleam or Draining Kiss), meaning you can't carry all three of Flamethrower, Surf and Psychic, which are needed to get passed Fairy resists.
But damn is it fast.
EDIT: Hell, even without any Speed investment Slurpuff hits 720, still faster than everything ever (watch out for them Scarf Deoxys lol). This allows you to relocate ALL of its Speed EVs into Defense to protect against priority, unless you are concerned about outspeeding certain targets unboosted.