Cobalion

Cobalion is one of the premier threats in the tier, with a versatile set of options including the ability to boost with both Swords Dance and Calm Mind, access to Stealth Rock, excellent Speed, and a great defensive typing that lets it check top-tier threats like Scizor, Hydreigon, and Bisharp. Its immunity to Toxic lets it come in on threats like Blissey and means it doesn't have to worry about losing to poison damage. Swords Dance is its flagship set, being very customizable and fulfilling a lot of roles besides that of a sweeper. Cobalion can run a variety of items to better fit the need of the team it is used on, with Z-Moves to wallbreak more easily, Rocky Helmet to punish physical attackers, and Shuca Berry to lure and remove Ground-types like Krookodile all being viable options. It is one of the best Pokemon at punishing Dark-types and Pursuit trappers thanks to its ability Justified, and its presence alone will make some of Cobalion's Dark-weak teammates able to be played more freely. There are some drawbacks to using Cobalion; notably, its Steel / Fighting dual typing leaves it without resistances to Psychic and Flying, which means that threats like Latias and Togekiss can be harder for Cobalion's team to deal with if it is the only Steel-type. It also faces competition from other offensive Stealth Rock users like Krookodile, Nihilego, and Terrakion, all of which have more unboosted power and different defensive typings to take advantage of. It also is severely lacking in initial power before boosting, having a mediocre base 90 in both Attack and Special Attack. This leaves the Swords Dance set walled by Pokemon like Slowbro even after a boost, while the Calm Mind set can struggle to get KOs on even offensive Pokemon. The Calm Mind set's main competition in Nasty Plot Lucario can wallbreak much more easily, as well.