Mew

Mew's niche in the UU tier stems from its diverse movepool and well-rounded stats, which allow it to fulfill the role of a stallbreaker or provide utility to its team in the form of Stealth Rock and Defog support. Mew's ability to outspeed most defensive Pokemon grants it multiple opportunities to get off a fast Taunt to limit bulkier Pokemon like Hippowdon, Pyukumuku, and Blissey from using their recovery moves or prevent Pokemon like Empoleon and Gligar from setting or removing entry hazards. Access to the combination of Seismic Toss and Taunt also allows Mew to beat almost every entry hazard user in the tier, including the likes of Empoleon, Rhyperior, and Mega Aggron, something very few Pokemon can claim. However, Mew is let down by its pure Psychic typing, which leaves it unable to effectively check a majority of offensive threats the UU tier has to offer, including staples such as Scizor, Bisharp, and Mega Sharpedo. This is worsened by its inability to carry coverage options outside of Seismic Toss and Knock Off effectively, making Mew extremely passive and easily exploited by threats like Infernape, Gengar, Facade Mega Altaria, and Hydreigon, the first of which it would be able to check with STAB coverage if its role permitted it. Mew is also susceptible to status moves like Toxic and Thunder Wave, which hinders it in its role as a stallbreaker and is especially troublesome for a Defog user, as its typing leaves it vulnerable to being worn down by every entry hazard. Mew struggles to deal with common threats like Hydreigon, Calm Mind Latias, and Celebi, as it is unable to punish them outside of Seismic Toss or status moves like Will-O-Wisp and Toxic, thus allowing the latter two to use Mew as a setup opportunity in most cases. Celebi in particular is able to shrug off all of Mew's responses through means like Recover, Giga Drain, and Natural Cure. Mew is also unable to run an offensive set, facing competition against other offensive Psychic-types such as Latias and Celebi, the latter of which, while sharing the same stat distribution, has a secondary typing to break through bulkier teams, unlike Mew.