In the Pokémon series, few types are as interesting as the Ghost-type. It boasts immunities to both Normal and Fighting moves, and it is super-effective against itself, which is a gimmick that it shares only with the Dragon-type. Pure Ghost-types are Pokémon that only have this typing, which can be harmful or beneficial depending on the situation. A way to measure the strength of these Pocket Monsters is through their viability in the main series games.
Information regarding these Pokémon is available on Serebii.net. Some Pokémon can change to Ghost-typing because of a held item or Ability/Hidden Ability like Libero or Protean, but these do not qualify as pure Ghost-types. Mega Evolutions, however, do count, because they are essentially different Pokémon altogether. Not every pure Ghost-type is meta-relevant, but certain ones can be helpful in the Pokémon games.
Cursola
With its uneven stat spread, Cursola requires strategizing to be able to use effectively. It can be viable sometimes in the competitive scene, but since the main series games usually do not need extensive planning to complete, there are better options when it comes to pure Ghost-types. It has an exceptional Special Defense stat, but its Defense is terrible. With an awful Speed stat as well, Cursola must get lucky and face a Pokémon that focuses on Special attacks. Its Ability Weak Armor lowers its already minuscule Defense but raises its Speed significantly when it is hit. However, to benefit from this Ability, it must survive a Physical attack, which it cannot do easily. Its Hidden Ability Perish Body, which has an effect similar to Perish Song, can be better, but it is not consistently reliable. Galarian Corsola has what some consider to be one of the worst shiny variations introduced in Generation VIII, and its evolution often disappoints in its own way.
Polteageist
Polteageist has similar downsides as Cursola, albeit to a slightly lesser extent. Its Defense stat is a bit higher but still not great, and its Speed, while only decent, is much better than Cursola's. This lets it move faster than more opponents and strike using its great Special Attack stat. It also has the Ability Weak Armor, which helps the Pokémon more because of its higher Defense. Its effect can make Polteageist's Speed viable and allow it to out-speed more enemies.
Its Defense stat is still mediocre, though, so there are times when Weak Armor does not help at all. The Hidden Ability Cursed Body can occasionally disable moves that hit the Pocket Monster, but it does not activate frequently. While not as weak as Cursola in a playthrough, Polteageist is still not always reliable.
Cofagrigus
Cofagrigus is an odd Pokémon. Its line's signature Ability Mummy imprints this Ability upon an enemy that makes a contact attack against Cofagrigus. While the Ability itself does not have any effect, it nullifies the opponent's Ability. Aside from this, Cofagrigus is capable defensively. Its Defense is fantastic and its Special Defense is fairly high as well. It rarely moves faster than foes, but these stats help it to cope. It can survive a variety of hits, and with its relatively high Special Attack stat, it can strike powerfully in return. Its low Attack stat limits it to only being strong against opponents with low Special Defense, but it can be very useful in these matchups.
Dusknoir
The final evolution of Duskull, one of the most underrated Ghost-type Pokémon in the series, Dusknoir has an interesting stat spread. It has great defensive stats, with its Defense and Special Defense sharing the same high value. However, its HP stat is abysmal. This puts Dusknoir in a complicated position. Its defenses let it take some decent damage, but not an excessive amount. Offensively, Dusknoir is mostly a Physical attacker, although its low Speed inhibits it. Dusknoir's defensive situation may be odd, but it usually allows it to withstand several hits nonetheless.
Mismagius
The Generation IV evolution to Generation II's Misdreavus, Mismagius gives the evolution line a much-needed improvement. With solid Speed and Special Attack stats, it can strike quickly and powerfully. Unfortunately, Mismagius does not have many strong moves available without TM or TR. Trainers must rely on these items or wait to evolve Misdreavus until it learns good moves.
Once it knows these moves, though, Mismagius can be more than capable as a Special attacker. On the other side, its Special Defense is solid, but its Defense, like that of many other Ghost-types, is underwhelming. Mismagius can be in trouble if it does not knock enemies out quickly, but it can do this often, so it is not a frequent issue.
Mega Banette
Banette has perhaps the scariest design in the Pokémon franchise, and its Mega Evolution continues this theme. Its defensive stats are only slightly better than those of its standard form, but its Attack stat undergoes a massive improvement. Its HP, Defense, and Special Defense are not great, but they can usually let Mega Banette survive an attack or two. As long as it does this, the opponent has little chance to stay in the battle. Its Speed is not exceptional, but if trainers want to use Status moves, Mega Banette's Ability Prankster increases the Speed Priority of these actions. It can also use Sucker Punch, a Physical Dark-type attack that strikes before the opponent. This only works if the foe is preparing to attack, but it is exceptional when it hits. Although Mega Banette does not gain a Same Type Attack Bonus from this move, its Attack stat more than makes up for it.
Spectrier
Spectrier is one of the most recent Pokémon added to the franchise. Even so, it stands tall among pure Ghost-types from throughout the series. It has an incredible Speed stat, and its huge Special Attack can let it sweep through enemy teams with ease. In addition, when it knocks out a Pokémon, its Ability Grim Neigh activates, giving its Special Attack stat a boost. This turns it into an even more powerful attacker. Its defensive stats are not impressive by Legendary standards, but they are still capable. Although the only Special attacks it can learn via leveling up are Hex and Shadow Ball, it can learn many more through TM and TR. Spectrier embraces its role as a Special attacker and does so exceptionally well.