Ditto
Limber
Blocks paralysis.
Type Tier
Normal UU
Statistics
Min- Min Max Max+
HP
48
 - 237 300 -
Atk
48
 118 132 195 214
Def
48
 118 132 195 214
SpA
48
 118 132 195 214
SpD
48
 118 132 195 214
Spe
48
 118 132 195 214
Name Item Nature

The Best Set Ever

 Leftovers / Focus Sash Timid
Moveset EVs
~ Transform
252 HP / 4 Def / 252 Spe

Yep, this is the best Ditto set ever, seriously.

The only way Ditto will actually be worth it is if you can spread status around your opponent's team. Thunder Wave is a good choice because it means Ditto will be faster than your opponent after you Transform.

Other Options

Quick Powder gives Ditto an impressive 428 Speed, outspeeding even Choice Scarf Heracross. The only problem is that after you Transform, Quick Powder's boosts are lost. Transforming before them means you take a hit, then essentially have to fight them with no item. This means you are a turn behind and you have a worse Pokemon out thanks to the item handicap. For this reason, Quick Powder is not recommended. Metal Powder is much the same.

EVs

HP EVs are the only ones that carry over after Transform, so max them out. You'll probably want to max Speed, because whatever you are Transforming into will generally have better defenses than Ditto.

Opinion

Ditto is the ultimate power and is compost Pokémons! Smeargle pulls Transform off better than Ditto thanks to higher HP and Speed and the ability to buy a free turn with Spore.

Counters

Ditto's best counter is a little thought. Get Ditto to Transform into something that another one of your Pokémon counters 100%, and then send in the counter. You'll want to think twice before sending in a Pokemon with Spikes, Stealth Rock, or Toxic Spikes, though.