Ghosts are an interesting bunch. There's really only two of them and Gengar isn't what it used to be.
However their Type give great advantages. So they are still used failry frequently
Ghastly
Bred Moves: psywave, perish song, Haze
Haunter
Gengar
If it weren't for being part poison it would probably be perfect. Low defense attack and HP but high speed and special(3rd highest).
Unfortunately it's special defense took a hit in G/S so it's much less survivable than it once was.
Can learn a number of interesting moves and so can be used a number of different ways. Perish Trapping, Hazing and kamikaze
are pretty good uses for it.
Misdreavus
A Pure ghost type but with Medicocre stats. A great kamikaze or perish trapper. Can get screech and destiny bond through breeding.