Moveset

The best Moveset for Ditto would have to be this:

Attacks:

Transform

Items Attached:

Metal Powder

With this Moveset, you are capable of transforming into any of your opponent's current Pokémon, aswell as maybe your team mate. The Metal Powder is used to boost Ditto's Special Defense.
Ditto's stats are average, but with any luck, and the correct Strategy you should be able to get it right.

Strategy Against Ditto

To get Ditto, you have to play a dirty trick and get him to Transform Into you and then use an attack which would be Super Effective on you, such as Rock Slide if you were a Entei. While this puts you at risk, if you are using a decent Pokémon with decent stats, you should survive if he tries it back on you.

Locations in Games

Ditto is only found in Pokémon Fire Red & Pokémon Leaf Green in the following Places: Cinnibar Mansion Unknown Dungeon Routes 13, 14 & 15. It is fairly rare so be prepared.

Animé Appearences

Ditto has only appeared in a couple episodes and when it has, it was always forced to be a part of a Team Rocket Scheme...first to Present a Dratini to the Boss, then to Trade it for a Rare Pokémon such as a Dragonite. Ash also beat one in the Orange League using its Transformation abilities against it with Pikachu's speed. It appeared in these episodes: Episode 37: Ditto's Mysterious Mansion Episode 113: Hello Pumello Episode 176: Imitation Confrontation