Shedinja

There is no Pokemon that is more polarizing, matchup-based, or irritating than Shedinja, as its very existence invalidates multiple common threats like Mega Medicham, Calm Mind Magearna, and Tapu Koko, as well as common variants of Manaphy and Mega Swampert. It has the unique quality of blocking both Volt Switch and U-turn, killing momentum from the likes of Magearna, Tapu Koko, and Mega Scizor. Its immunities can also be put to use against Choice-locked attackers like Tapu Lele and Ash-Greninja, taking advantage of these foes by spreading status. Due to how Shedinja can singlehandedly remove offensive counterplay and generally bug common offense teams, it finds a strong niche on certain stall teams. However, in the same dramatic fashion that it voids these Pokemon, Shedinja is rendered useless when facing sand teams or when Stealth Rock, Spikes, or Toxic Spikes are on the field. Additionally, Shedinja is more Pursuit-weak than most, with only one escape afforded to it by its Focus Sash. The biggest nail in the coffin is how prevalent Toxic and Leech Seed are on balanced and defensive teams, meaning that Shedinja is generally only useful against certain Pokemon on those teams, like Mega Latias and Swords Dance Gliscor.