Hoenn did not introduce a ton of Ghost-type Pokemon, but the ones it did are a great addition to the Pokedex.
The Gastly line, including Haunter and Gengar, were pretty lonely in the Ghost-type category. Johto didn't really make up for that, but Hoenn added more than a few to the series.
While the Hoenn Ghost-type Pokemon are not as iconic as Gengar, they are pretty useful in a playthrough of Generation III. They are also fairly terrifying, as most ghosts are in real life.
Top 3 Ghost Pokemon from Hoenn
#3 - Shedinja
Shedinja is a strange and mysterious Pokemon. In fact, many players completely missed out on Shedinja the first time around during Generation III. That is due to it only being obtainable by evolving Nincada with an open Party slot. An extra Poke Ball was not needed in its debut Generation.
Its base stats are terrible, with its highest being a 90 Attack. The uniqueness comes from its Ability, Wonder Guard. It prevents attack damage from being done unless it is supereffective. That is a scary thought for trainers with no Dark, Fire, Flying, Ghost, or Rock-type moves.
#2 - Sableye
Sableye is a pretty tricky Dark/Ghost-type Pokemon. It has jewels for eyes and is said to steal the soul of any person who stares into them. That is terrifying to say the least.
In terms of battling, it has pretty low stats. Its usefulness is due to its trickiness. It can set up, learn moves that allow it to attack first, or attack last with immense power. It only got better as time went on, but in Generation III, it was a solid Ghost-type partner.
#1 - Dusclops
Dusclops is known for its bulkiness in more recent competitive settings, especially with an Eviolite. In Hoenn and Generation III, Dusclops did not have Dusknoir to look forward to. It did have massively high Defense and Special Defense at 130 each.
It is known to hypnotize people and Pokemon, forcing them to do its bidding. Also, it sucks the energy out of will-o'-the-wisps into its own body. The bulk and the lore make it the best Ghost-type from Hoenn.