The Underused Banlist (UUBL) tier of competitive Pokemon is the list of Pokemon from each generation that aren't used enough to be considered for the Overused Tier, but also don't qualify for the Underused Tier or below. Underused is defined as not having a 3.41 percent usage in Overused. It's an unplayable tier, but can be a great reference for building a battle list for in the Uber or Overused tiers.
The Pokemon on the UUBL are considered too overpowered for lower tiers, but still pair well with higher-tier strategies. They've all got something special about them, though, even if they're not always the first choice.
10 Gyarados
While Gyarados has always been a fan favorite, it can be a touch-and-go Pokemon to use in competitive battling. Its high Attack stat and Dragon Dance make it a good sweeper, but it can be taken out quickly if it doesn't have a chance to set up. Its Speed stat is fairly low, meaning it's usually doomed to go second against speedier Pokemon.
Gyarados is good for either early-game wall softening or late-game cleaning. The best choice for partners with it are ones that are stronger against Electric-types, as Gyardos' Water/Flying dual-type makes it extremely vulnerable to Electric moves.
9 Ninetales-Alola
Alolan Ninetales, equipped with Aurora Veil and Snow Warning, is the only viable Pokemon with access to the former, along with the ability to use Hypnosis or Encore. These traits, combined with its high speed, make it a great early-game sweeper against setup Pokemon. Aurora Veil halves the damage the team takes, and Freeze-Dry will cancel out rain attacks.
The drawbacks come from it being an Ice-type, which, when combined with its Fairy-type, makes it extra weak to steel. Ninetales-Alola is an iffy choice sometimes, but your team won't be lacking if you do decide to bring it along.
8 Blaziken
Swords Dance. Two words that strike fear in the hearts of players. This devastating move ups your Pokemon's Attack by two stages, maxing out at six stages. This can make a Pokemon with an already high attack absolutely deadly.
You'd think this would make Blaziken an easily overused Pokemon, but unfortunately, it can have a hard time finding setup opportunities because of its average Speed. Put it with the right team, however, and Blaziken will shine. The variety of its attacks - Flare Blitz, Close Combat, and Thunder Punch - are a huge benefit, especially if it can get the first attack.
7 Hawlucha
Hawlucha is an offensive fighter with a lot going for it. It has Swords Dance for boosting up its attacks, and a naturally high Speed that is almost unmatched by any other Pokemon. It's best as a late-game choice, with strong teammates to carry it, as its defense and HP are on the lower side.
It really only gets one chance to sweep - if the other team can get an attack in, it can wipe Hawlucha out. This has to do with the fact that it's vulnerable to all status effects, it can be KO-ed by some common OU tier picks, and its status boosts can be canceled out by abilities like Haze. While Hawlucha is good, it's best to save it for when you know you can make up for the shortcomings.
6 Gengar
Gengar's high Special Attack and Speed already make it a force to be reckoned with. Add in Nasty Plot to up itss Special Attack even more, the natural immunity to moves like Toxic and Seismic Toss, and Substitute to avoid most other status effects, and Gengar can seem unstoppable.
This makes it a veritable powerhouse, which fits in with OU tier Pokemon like Tapu Koko, Rillaboom, and Weavile. These partners pair well with Gengar and make up for its few shortcomings. Overall, it can be a must-have 'mon for offensive and hyper offensive teams.
5 Mienshao
Mienshao thrives against physically weaker Pokemon with lower defense stats. Its move set includes Close Combat, a move capable of KOing or 2HKoing most opponents, U-Turn, which allows it to attack and immediately switch out if it's against a stronger opponent, and Knock Off, which will take out the enemy's items.
High Jump Kick or Poison Jab are the ideal fourth moves, High Jump Kick making up for Close Combat's low PP and Poison Jab offering a 30 percent chance to poison the opponent. It falls down against physically defensive, Ghost, and faster Pokemon, as well as 'mons with Priority moves.
4 Alakazam
Alakazam is another speedster with a high special attack, a valuable ability, and a moveset that makes it a must-have. Equip it with a Life Orb and go in with Psychic, Focus Blast, Shadow Ball, and Hidden Power Fire or Ice. The Magic Guard ability protects it against direct damage, which is important for its low defense.
Psychic is its main move, with Focus Blast acting as a backup against Dark- and Steel-types. Its great moveset makes up for its low defense and low HP, and you'll want to make sure you're not switching in on attacks, because Alakazam can be one-shotted, and you don't want to lose it too early on.
3 Weavile
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Weavile is an offensive Pokemon through and through. Its high Attack stat, paired with its high Speed, make it formidable and easily capable of fitting in with any other type of team. It's unfortunately weak against common types such as Fairy, Fighting, and Steel, as well as a weakness to stealth rock.
It still makes for a good early-game choice to take out potential checks with Knock Off. It can also be good for a late-game sweep, or a revenge kill. Try to avoid switching it in and out too much if Stealth Rock is in play, and always try to bring it in on Volt Switch or U-Turn.
2 Dragonite
Dragonite equipped with Flyinium Z can be a true threat to opponents. Dragon Dance and Supersonic Skystrike can break through most defensive staples, and the Multiscale ability makes sure it has an easier time setting up.
Unfortunately, Dragonite's Speed is fairly low, leaving it vulnerable to revenge kills, and requires support from the rest of the team to keep from being taken out. It is especially vulnerable to being Paralyzed, and it can easily be out-speed. It is mostly a Pokemon for hyper offensive teams.
1 Aegislash/Aegislash-Blade
Aegislash's dual Ghost/Steel-typing is what makes it a top contender, with its immunity to some of the heavily used attacks. Its two forms - Base and Blade - have high Defense/Special Defense and high Attack/Special Attack respectively. The ability Stance Change allows it to switch back and forth, which can make it hard to counter.
Conversely, you can give up the Defense/Special Defense to become a terrifying offensive attacker. Throw in a Spell Tag to power up Ghost-type attacks, and Aegislash will blow right through early- to mid-game Pokemon with Shadow Ball and Close Combat for opponents immune to Shadow Ball.