Ditto

Ditto can automatically transform into the opposing Pokemon upon switching in, copying their moves, nature, EVs, and any stat boosts. However, Ditto does not copy their HP, meaning that it has to live with a horrible base 48 HP. Ditto also can't transform into Pokemon behind Substitutes. However, the main draw of using Ditto is that it can transform into an opposing sweeper and take their boosts to turn the situation against the opponent, a strategy that appreciates Choice Scarf, which can make sure that Ditto outspeeds its foe. Ditto still loses to most Calm Mind or Bulk Up sweepers, as they can beat Ditto one on one with their higher HP stat. Ditto is almost completely useless against stall teams besides sometimes using utility moves such as Defog or Heal Bell. Another problem Ditto faces is that the moves it copies only have 5 PP, meaning it can be PP stalled, especially as it almost always holds a Choice Scarf.