The determination of the "best" Pokémon of any given type or category is always subjective because the attributes a trainer seeks determines which Pokémon is the best fit. However, there is a strong case for Giratina being the most useful Ghost-type Pokémon.
The Ghost type has always been considered one of the best defensive types in the game, possessing resistance to four other types, immunity to trapping moves and the ability to run from any Pokémon battle regardless of speed. Giratina is a Ghost-and-Dragon type legendary Pokémon that learns the strongest Ghost type move, Shadow Force. It also has a base stat total of 680, which is the largest of all Ghost-type Pokémon by a large margin. With all of these factors considered, Giratina may be the most useful Ghost-type Pokémon.