In my opinion, it's Mimikyu. Its Disguise ability can take a Z-move for free, which is insane. It also has a great movepool, Swords Dance, and great all-around stats.
Mimikyu @ Life orb
Nature: (doesn't matter but preferably jolly or Adamant)
- Play rough
- Shadow Claw
- Shadow Sneak
- Drain Punch
As Master had said its type overlaps with decidueye so if you care about type overlaps I would have picked Rimbombee or Comfey. But Rimbombee is found late and Comfey has a bad movepool but then shiinotic came. Its has a pretty decent movepool with ok stats but it works
Shiinotic @Leftovers
Nature:Again doesn't matter but I prefer modest or jolly.
-Moonblast
-Giga drain
-Spore
-Strength Sap
Hope I helped!