Shedinja

Shedinja is far and away the most unique Pokemon in the game. It has just 1, yes 1, HP, but it's protected by its ability Wonder Guard, which makes Shedinja completely immune to any damage that isn't a super effective attack or passive damage. While immunity to a whole slew of attacks sounds awesome and would make Shedinja one of the premier walls in the game, it is unfortunately KOed by literally any passive damage, which is rife in today's metagame. Shedinja's stats are also pretty unimpressive; it's extremely slow and has a terrible movepool which undermines its good Attack stat.

Shedinja is hard to use, but it does have a valuable niche in the metagame; it's one of the best counters available for Water-, Grass-, Poison-, and Electric-type Pokemon that lack Toxic or a super effective attack, which normally includes Pokemon such as Milotic, Venusaur, Leafeon, Exeggutor, Lanturn, Slowbro, Azumarill, Feraligatr, and Cloyster, among many others. Shedinja is the definition of high-risk high-reward, and if you can keep it away from Ghost-, Fire-, Rock-, Flying-, and Dark-type attacks, as well as burn, poison, sandstorm, hail, Leech Seed, and entry hazards, it can prove to be the ultimate "win condition" Pokemon.