You want broken? HERES SOME BROKEN STUFF! (would also be nice if Luvdisc or Farfetch'd were on here, but the latter does have its Stick, so you can't win all the time).
Fake Bonsai Tree: When held by Sudowoodo (and Bonsly), its Speed is quadrupled and Water-type damage is fully negated.
Sneaky Gift: When held by Delibird, all stats (barring HP) are doubled. Present is now considered Ice-type and will always hit for 120 BP and heals the user's max HP by 25%.
Violin Spear: When held by Kricketune (and Kricketot), its Attack and Speed are doubled and its attacks fully pierce through the opponent's defenses (Protection, stat boosts, screens, Substitute, and any other thing that reduces damage).