Misdreavus

Misdreavus is the only pure Ghost-type available in GSC OU, which gives it an immediate niche. Its typing grants it a coveted Normal immunity, allowing it to pivot in on a predicted Explosion, as well as check most Snorlax variants and prevent Starmie from using Rapid Spin much more reliably than Gengar. Moreover, Misdreavus has many tricks up its sleeve thanks to its movepool. Its Perish trap set is one of the main reasons why so many teams pack a phazer, and such teams still have to worry about the Disruptor set, which can badly cripple them.

Nonetheless, Misdreavus is frail and lacks significant firepower, which means it can be quickly overwhelmed by most offensive powerhouses without doing anything significant beforehand. It also needs to watch out for Pursuit and faces competition from Gengar, which is much more immediately threatening and is immune to Toxic, a move commonly used by Cloyster and Forretress, two Pokemon you typically want to check with your Ghost-type.