Genesect

Genesect's diverse movepool, paired with its great mixed offensive stats and access to power-boosting abilities, means it can easily plow through slower teams and makes it very difficult for faster Pokemon to switch in. Genesect has access to Extreme Speed, which, paired with a stat-boosting ability, makes Genesect a very potent revenge killer, and sporting the second strongest U-turn in the AAA metagame, Genesect can easily maintain offensive pressure and gain momentum while taking out huge chunks of the foe's HP when it U-turns out. Genesect's typing also grants it a nice set of resistance and a single weakness to Fire-type moves.

However, Genesect often finds itself being outpaced and overpowered against hyper offensive teams, and full stall teams usually carry both physical and special walls that can wall different Genesect sets and wear it down via entry hazards. Other than that, Genesect variants that lack Extreme Speed are always forced out by the ever-present Gale Wings users and, bar variants that are immune to Fire-type moves, Fire-types also generally force Genesect out. Another downside to using Extreme Speed Genesect is that it's forced to use a Hasty nature due to Extreme Speed being an event-exclusive move. However, Genesect can effectively beat the checks and counters of one set with another one, making it very unpredictable and powerful. That, along with its ability to overpower a lot of the common walls or revenge kill a good amount of the metagame, makes Genesect easy to fit into most teams.