Crobat

Crobat's great Speed stat allows it to be an effective revenge killer capable of picking off threats such as Celebi, Infernape, and Gengar, and its ability Infiltrator allows Crobat to bypass Substitute and revenge kill Gengar even through its Substitute. Crobat is also a decent stallbreaker, as it can annoy and whittle down slower Pokemon such as Alomomola, Mega Aggron, and Blissey with the combination of Taunt and Super Fang. Furthermore, an immunity to Toxic prevents these Pokemon from being able to break past Crobat. Crobat's Speed and typing also allow it to be a great check to Dragon Dance Mega Altaria and consistently pivot into Ground-types like Gligar and Hippowdon. However, while Crobat has a stellar Speed stat, it is easily revenge killed by the faster threats in the tier such as Zeraora, Mega Aerodactyl, and Mega Manectric. Mega Aerodactyl in particular is able to shrug off Super Fang damage with Roost and proceed to Pursuit trap Crobat. Crobat also has a mediocre Attack stat, which prevents it from being able to revenge kill threats like Latias and Terrakion, especially if Crobat has expended its Z-Move. Crobat's weakness to Stealth Rock greatly hinders its ability to pivot into Pokemon like Cobalion and Mega Altaria effectively, thus increasing the need for Crobat to use Roost to stay healthy, which can be a huge momentum drain.