One of the biggest game-changers added in the third generation of Pokemon was abilities. Each Pokemon has an ability that grants them a passive effect during battle or in the overworld. There are many varieties of ability - some are hidden, some change on evolution, some can even be detrimental. One thing that the games used to set certain species apart are signature abilities - ones that belong to a single evolutionary line.
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Signature abilities aren't powerful by definition, but plenty of the best abilities in the series are signatures. This list doesn't include abilities that used to be signatures, but later got added to multiple lines. For instance, Sand Stream was once exclusive to Tyranitar, but it would eventually be shared by Hippopotas.
10 Wonder Guard
Arguably, Wonder Guard should be at the top of this list, not the first entry. This ability nullifies any attack that isn't super-effective. For example, if a Water-type had Wonder Guard, it could only take damage from Grass and Electric-type moves. What could be better than that?
The problem is that Wonder Guard is exclusive to Shedinja. Shedinja has one hit point, meaning that any super-effective attack will drop it immediately. Bug/Ghost isn't the greatest defensive typing, with five weaknesses. The ability doesn't protect it from burn or poison damage, either.
9 Steam Engine
The ability is exclusive to Coalossal and its previous stages Rolycoly and Carkoal. A Fire/Rock-type like Coalossal is going to have a pretty hard time with Water-type moves, which deal quadruple the damage. Luckily, the line has Steam Engine to cover for it somewhat.
When hit with a Water or Fire-type attack, Steam Engine boosts the owner's Speed six stages - up to the maximum for stat boosts. Coalossal is a very slow Pokemon, so it appreciates some help. However, it doesn't mitigate the massive damage done by the attack. It might be better to keep Coalossal out entirely instead of risk it to get this ability to activate.
8 Cotton Down
Exclusive to Gossifleur and its evolution Eldegoss. Whenever a Pokemon with Cotton Down is hit by a damaging move, it explodes into cotton fluff, lowering the Speed of all Pokemon aside from itself. This will trigger every time it's attacked, and multi-hit moves like Fury Swipes will set it off multiple times.
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Note the phrasing - all other Pokemon. In a Double or Triple Battle, Cotton Down will also activate against Gossifleur's allies, reducing their speed. Sounds horrible, and most of the time, it is. However, this could be an interesting way to run a Trick Room team in Multi Battle.
7 Gale Wings
A Hidden Ability only found on Talonflame and its previous stages. All Flying-type moves from the user take priority, meaning they automatically go first. That's huge. Talonflame might not have had the best stats, but simply having Gale Wings made it very popular in competitive play.
It seems Nintendo decided they made Gale Wings too good, because come Pokemon Sun and Moon, it got a considerable nerf. Its ability now only takes effect when Talonflame is at full health.
6 Disguise
Mimikyu uses its handmade Pikachu disguise for its signature ability. The first attack to hit Mimikyu breaks its costume and turns it into the Busted Forme. That attack doesn't do any damage as a result. It's simple but effective.
Here's a little-known fact: Using Rollout on a Mimikyu used to trigger a glitch. Rollout works by incrementing a turn number and a damage multiplier every round. If one of those hits busted Disguise, the turn number would go up one as usual, but the damage multiplier wouldn't. So, after Rollout was finished, the user would have one more leftover turn where they had a huge damage boost for any move they wanted.
5 Mummy
The signature ability of Yamask and Cofagrigus. When a Pokemon with Mummy is hit with a move that makes contact, the attacker's ability is erased and replaced with Mummy. Not all abilities can be changed this way, but the vast majority can. It's very useful to shut down annoying abilities.
Be very careful when using Mummy, as it can backfire easily. There's a decent chance the opponent you just Mummified will spread the attack back to your own team. Switching out will not change the ability back. It's a dangerous game to play.
4 Pure Power
A powerful signature ability exclusive to Meditite, Medicham, and Mega-Medicham. A Pokemon with Pure Power's Attack stat is essentially doubled. This ability works through directly modifying the stat instead of the typical boost from moves. So the Attack stat can grow even more through moves like Swords Dance.
Technically, this isn't an exclusive ability. Yes, Pure Power is only found on the one evolutionary line. However, there's another ability called Huge Power which is functionally the same thing. Both abilities double the Attack stat. Huge Power is found on the Marill line and Mega-Mawile among others.
3 Stance Change
This ability makes Aegislash what it is. The Pokemon starts out in Shield Forme, with high Defenses and low Attack. When it uses an attack, the stats swap, with high Attack and low Defenses. It can revert back to Shield Forme by using its signature move King's Shield. Switching out also causes it to revert.
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Stance Change is an extremely exclusive ability. It can't be copied by Trace, swiped with Skill Swap, or be replaced by Mummy or Worry Seed. Even a Ditto that has Transformed into Aegislash and copied its ability won't be able to change Formes.
2 Parental Bond
Parental Bond is exclusive to Kangaskhan's Mega Evolution. When the baby jumps out of its pouch and joins the fight, almost every attack gains a second hit. The extra hit does 50% of the damage of the first, and yes, the extra hit can be critical. Kangaskhan is already a pretty powerful Pokemon, so this ability makes it one of the hardest hitters around.
Unfortunately, Mega-Kangaskhan didn't make it to Pokemon Sword and Shield, so Parental Bond isn't available anymore. Perhaps the little tyke needed a nap.
1 Libero and Protean
Libero and Protean are functionally the same ability - the former belonging to the Scorbunny line and the latter to the Froakie line. Immediately before using any move, the Pokemon's type will change to match the move. This is triggered by status and support moves as well as attacks.
The obvious benefit from this ability is that the user will always receive same-type attack bonus on every move. Protean also has some defensive utility. Constantly changing type will make it hard for the opponent to get super-effective hits in.