Held: Increases the holder's Speed by 50%, but restricts it to the first move it uses until it leaves battle or loses this item. If this item is swapped for another Choice item via Trick or Switcheroo, the holder's restriction is still lifted, but it will again be restricted to the next move it uses.
(Quirk: If the holder is switched in by U-Turn and it also knows U-Turn, U-Turn becomes its restricted move.)
Description: Literally anything that tries to set up with Swords Dance, Shell Smash, Nasty Plot, Agility, Rock Polish, Work Up, Tail Glow, or Dragon Dance are cloned and turned on their creators. While Ditto can't as easily beat Calm Mind, Quiver Dance, or Bulk Up Pokemon it can still put some of them under pressure.
Imposter effectively means that at any given moment you are a copy of the Pokemon you were facing when Ditto first entered the field. Besides your HP and held item Ditto becomes your opponent, including all its stat changes. This is where the Choice Scarf come into effect. Imposter will copy your opponent's stats but Choice Scarf guarantees you will be faster than them (unless they also run Scarf, in which case you speed tie).
Options: Though Ditto has the same or better in almost every stat, its HP does not change. Max HP EVs gives you 300 HP which isn't too shabby.
Though Ditto copies the stats ignoring its own non-HP IVs, it does not copy Hidden Power. What this means is you must pick your Hidden Power before the battle. It won't affect your stats but it's better than getting stuck with HP Dark. The best choices are Ice, Fire, and Rock as they hit the most Pokemon hard.
Counters: As Ditto transforms into your own Pokemon the only thing you need to do against it is play smart. You will know what moves it will run and most of the time it will be locked into one move. So don't be foolish and have your Terrakion Swords Dance when Ditto can copy it and sweep your team. Toxic Spikes will beat Ditto as even if it Transforms into a Steel / Poison / Levitating / Immunity Pokemon it will stay Poisoned.
This is a good moveset for ditto (Pokemon #132) with the imposter ability/trait, a Serious nature, and equipped with Choice Scarf submitted by Cybermewtwo. For use in competitive Pokemon battles featuring an Export option and breeding guide.