Ghost Pokémon are quite interesting, but Gengar is clearly the best out of them. YES IT IS MORE USEFUL AND BETTER THAN DUSCLOPS.
Shedinja is handy too, moreso than people seem to think.
It can stop many Pokémon cold, especially most legendaries' with their standard movesets (ex. Mewtwo, Latias, Latios, Kyogre) and remove all their wonderful Calm Minds and other stat-ups when they are forced to switch.
Just keep it away from a Tyranitar or Spikes using team, if you know your opponent has them.
Oh and give it Protect, in case you don't know whether your foe can hit you or not.
Sableye is probably the worst fully evolved ghost (Haunter is better than it), though I won't deny it's an interesting Pokémon.
Gengar's your best choice, it is one of the most versatile and unpredictable Pokémon ever, as well as one of the best (non-legendary) Pokémon available.