1. Gengar
What other Ghost type would be number 1? Gengar is an amazing ally. he has high attack and special attack to help with those annoying psychic types.
2. Sableye
Literally had no weaknesses until generation six, this Pokemon was amazing. With its dual type being ghost and dark it helped against psychic types, ghost types, fighting types, and normal types.
3. Aegislash
Probably one of the best Pokemon in the newest generation. with its high attack and its ability stance change it made this Pokemon a great wall including the move king's shield.
4. Banette
Banette has probably on of the most evil Pokedex entries ever. it revives from a neglected doll to hunt the doll's old owner!
5. Chandelure
Chandelure has an awesome dual type with fire and ghost making its special attack insane.