Which ghost pokémon is the most useful in single player?

I'm in doubt between Froslass and Sableye. I mean, during my last playthrough I used Sableye after the championship. I also brought in my old sableye through home.

But now I'm thinking Froslass might be good too, especially with the grass type gym coming up.

Naturally other options can be suggested too, but these are my main contenders.

So what do you think?