Ghost-type Pokémon are interesting. Way back in the days of Generation I, there were only three Ghosts in the entire game—Ghastly, Haunter, and Gengar—and all three were also part poison. In addition, their moves were only super effective against other Ghost Pokémon, making them questionably useful at best.
But as later games came it, Ghost became the primary type used for taking down the notoriously powerful Psychic type (back before the days of Dark-type Pokémon). In the anime, they even make Ash go get a Haunter for the express purpose of taking down Sabrina. Nowadays, Ghost Pokémon make up some of the most powerful monsters in the entire game, and we're counting down the strongest of them.
Shedinja gets an honorable mention because of its very unique defense. It has an ability no other Pokémon has called Wonder Guard. This ability prevents any move that isn't super-effective from hitting Shedinja at all. Its typing is Bug/Ghost so there are a fair amount of moves that can still hit it, including Rock, Dark, and Fire, but everything else passes right through this little insect.
However, it only has one single hit point, so any move that hits is a sure KO. You get this Pokémon using a strange method. It's the result of the only evolution that turns one Pokémon into two. When Nincada evolves into Ninjask, if you have an free party slot you get a Shedinja, Nincada's empty, cast-off shell.
What happens when you trap 108 restless spirits inside of a rock with a scary face on it? You get Spiritomb, of course! This Pokémon is Ghost/Dark type, which makes it completely immune to Fighting, Psychic, and Normal moves. It also has great defensive stats and pretty good attack, with a lot of moves that make Spiritomb a good support team member. However, it doesn't have a lot of HP to back up it's defense and no useful way to recover health, which limits it's overall usefulness. There are stronger Pokémon to come.
This imposing goliath of a Pokémon might not seem like the best choice at first. It's part Ground-type, which makes it weak to common types like Water and Grass, and it's Defense stats are nothing to write home about. Maybe it's a little fragile; it even has a bandage stick together a crack in its chest!
But what really makes Golurk stand out is it's incredible physical Attack stat. Plus it has an ability that boosts all of its "punch-based attacks". With an absolutely devastating Shadow Punch and immunity to Fighting and Electric moves, Golurk's a good addition to any team.
We think this might just be some sentient algae possessing some bits off a shipwreck, but that doesn't mean it can't be formidable in the right hands. Dhelmise has good Defense and access to the self-healing move Synthesis, making it easy to keep it in play for a long time. It also has access to Rapid Spin, a move that rids the field of all environmental hazards like Stealth Rock, which are all over competitive play. Add in to that a truly incredible Attack stat, and Dhelmise is ready to go head to head (does this thing have a head?) with even the sturdiest Pokémon.
Everyone's favorite little creepy Pikachu stalker doesn't look like a very powerful Pokémon. It's made itself a little costume to appear as sweet and likable as possible, and it might lead you to think there isn't a lot going on under the disguise. But Mimikyu is actually shockingly useful and its disguise actually works to its benefit. In fact, its ability is called Disguise and allows Mimikyu to take one free hit where the stuff Pikachu head absorbs all damage.
It also has the unique Ghost/Fairy typing which lets it use powerful STAB (same type attack bonus) moves on every other type of Pokémon for at least neutral damage. Of course, once Mimikyu's disguse is broken, it doesn't have a lot of HP to it's name, so it'd be wise to take it out quick before it can whip up something deadly.
Cofagrigus has an absolutely massive physical Defense stat and it's Special Defense is nothing to sniff at either. Actually, that makes sense considering it's hiding most of its substance inside a sarcophagus. It's a pure Ghost-type too, meaning it's only weak to Dark-types and other Ghost Pokémon, while being completely immune to Normal and Fighting. What really gives Cofagrigus an offensive edge is that it can use the moves Nasty Plot—which sharply boosts its Special Attack—and Trick Room—which affects the Speed stat of all Pokémon on the field. Fast Pokémon become slow, slow become fast, and Cofagrigus becomes terrifying.
This spectral chandelier is an offensive force to behold on the battlefield. It has a beastly Special Attack stat. That plus its dual type of Ghost and Fire give it monstrously powerful STAB moves like Shadow Ball and Flamethrower. It's only real downside is that it has a less-than-ideal Speed, which means its usefulness as a sweeper are limited. Still, it's a very dangerous Pokémon to encounter on the field. And it's scary purple flames don't even burn your body. They burn your soul, which just...isn't cool, Chandelure.
Being Steel/Ghost-type grants Aegislash an impressive amount of resistances and immunities right off the bat, but that's not what makes it so formidable. No, that would be it's unique ability Stance Change, which lets it swap between its Blade Forme and Shield Forme. In Blade, it sacrifices defense for 150 in both Attack and Special Attack, while in Shield it does the exact opposite. And since its an ability and not a move, it does this stance change automatically whenever it uses an offensive move or it's signature move, King's Shield.
This makes Aegislash an extremely powerful and dangerous attacker that's also incredibly hard to take down. If a rival Pokémon even tries to attack it after its used King's Shield, that Pokémon has its attack drastically lowered. How dare it even think about laying a claw on Aegislash?
First of all, let's just start with this: Mega Sabeleye is banned in most competitive play tiers. That on its own should give you an idea of how strong it is. Regular old Sabeleye isn't bad, especially with its Dark/Ghost-typing whose only weakness is Fairy. But it is still a single-stage Pokémon, and those are rarely stand-outs with regard to strength. Mega-evolving it gives it massive Defense and Special Defense, probably because of the gigantic gem it's hiding behind. That, in addition to its ability Magic Bounce, which rebounds status moves and environmental hazards onto the user, make it almost entirely overpowered.
Here he is. The original Scary Boy. Gengar was already a powerful Pokémon long before Mega Evolutions came around, and they went and made it into a force of nature. Something you would be afraid to check under your bed for. The ultimate Ghost Pokémon. It has a Speed of 130 and a Special Attack of 170. It's Ghost/Poison-type, so it's immune to Normal and Fighting, and can go up against passive enemies that rely on moves like Toxic or Toxic Spikes. And it's also the only Ghost Pokémon to have the ability Shadow Tag, which prevents opposing Pokémon from switching out of battle. Mega Gengar isn't trapped in here with you, you're trapped in here with it.