Ditto

Ditto carves a small, yet unique, niche in the Ubers metagame: its ability Imposter allows it to completely copy the foe, including stats with the exception of HP, ability, and any boosts that have been acquired. When equipped with a Choice Scarf, Ditto is able to completely deter setup on the opposing team and revenge kill Pokemon that have already acquired boosts, such as Xerneas, Rock Polish Primal Groudon, and weakened Extreme Killer Arceus. Furthermore, Ditto doesn't necessarily have to revenge kill - it can scout for moves using Imposter to potentially give you the advantage. However, Ditto can easily be played around. Because Ditto copies the opposing Pokemon's moveset, the opponent can potentially predict what Ditto will use and play around it. Furthermore, Ditto is completely useless against stall teams and support Pokemon such as support Arceus formes and Mega Sableye, as it can't make use of their moves well; it is usually only effective against hyper offense, where the opponent does not have checks for everything. Ditto is also ineffective against Substitute users, as it cannot transform into a Pokemon behind a Substitute. To compound this all, Ditto's low HP means that it can easily be worn down.