Ghost types have immunity to Fighting (and Normal) type attacks, making them a good pivot for Normal/Steel/Dark/Rock/Ice types, and good counters to Hyper Beam and Explosion. In Gengar's case, it is also immune to Ground, and gets Thunderbolt and Ice Punch, making it a huge threat to Fighting and Ground Types, as well as resisting or immune to more Physical Types than not. Gengar managed to stay in OU for generations 1, 2, and 3, so at least Gengar is worth using. :P
Also, Dusclops, Sheninja, Sableye, and Banette have higher Attack than Special Attack, but they are only in Gen 3. :P