In the history of the Pokémon series, only three types were not a part of the original fifteen in Generation I. One of these is the Dark-type. Introduced in Generation II, Dark changed the landscape by countering the overpowered Psychic-type. Some Pokémon only have this typing, making them pure Dark-types. One can measure their power through their main series game capabilities. Serebii.net has stats and other information that help to rank these Pokémon.
While some Mega Evolutions qualify as pure Dark-types, Pocket Monsters like Arceus, Greninja, etc., that can change to any typing with an item or Ability/Hidden Ability do not count. The Dark typing has made a name for itself in the main series games, and if trainers can find a powerful Pokémon of this type, it can help them substantially.
Mightyena
Mightyena is useful in the early portions of the games, with a relatively high Attack stat and decent Speed. Mightyena also has the Ability Intimidate, which cuts the Attack of opponents when it enters the battle. Its Hidden Ability Moxie can also help in trainer battles with multiple Pokémon. These benefits, however, do not make up for the barriers it faces as the games continue. Its base stat total is not high enough to compete with late-game threats as well as others can. Even though Team Magma and Team Aqua, the villainous teams of Hoenn, use Mightyena consistently, it becomes less frightening as the plot moves forward.
Thievul
With a high Speed stat among other early-route evolutions, Thievul is able to move faster than a large number of opponents. It has a decent Special Attack stat, but it cannot make much use of it. The only Special move it learns from leveling up is Snarl, which is unimpressive compared to other Dark-type attacks. It has above-average Special Defense but low Defense. As a result, it can faint easily with an unfortunate matchup. Its Special Defense is not high enough to make this tradeoff worth it, leaving Thievul behind others when it comes to late-game team members.
Liepard
Although Liepard's base stat total is slightly lower than Thievul's, its stat spread makes it more viable. It is fast, and its offensive stats are decent, at least by early-game standards. When in neutral matchups against weaker enemies, Liepard can sweep through teams with these stats. As time progresses, though, this becomes increasingly difficult.
If it cannot knock opponents out in one hit, Liepard is at a disadvantage. Its defensive stats and HP are low, making it easy to take out. Players can use it effectively for a portion of the game, but there are eventually better options. Regardless of its limited abilities, Liepard is one of the more unique-looking Pokémon whose character design stands out in its Generation.
Alolan Persian
Alolan Persian is an interesting Pokémon. It is considered one of the worst-designed regional forms in the series, but it has its place on players' teams. At first glance, Alolan Persian may appear very weak. Its base stat total is underwhelming, and its stat spread leaves it mediocre in most areas. However, it gains a significant boost from the Ability Fur Coat. This halves the Physical damage it takes, which essentially doubles its Defense stat. Alolan Persian still struggles against Special damage, but its Ability makes it much more capable. It rarely carries players through the games, but it is not the worst pure Dark-type around.
Zoroark
Zoroark has a fascinating Ability. Illusion lets it take on the identity of another Pokémon in the party, confusing opponents. Enemies usually attempt to use a super-effective attack against it, which trainers can strategically manipulate. This confusion can give Zoroark a mostly free turn to use its great offensive stats, which can overwhelm some Pokémon in battle.
It also has a high Speed stat, so it can often strike twice before an opponent can counter it. Once this gimmick plays out, though, Zoroark can fade quickly, having low defensive stats and HP. However, in the time it is on the battlefield, it can be dominant as any other Pokémon of this type, and is good to use for players who like to strike early and fast.
Umbreon
The Dark-type evolution to Generation I's Eevee, Umbreon had a reputation to live up to when it entered the franchise. Many years later, it still ranks as perhaps the best Eeveelution. It has incredible defensive stats and a high HP stat. While its Speed, Attack, and Special Attack are lacking, Umbreon's bulk keeps it in battles longer than many. It can use this time to wear opponents down and use some of the great moves it can access. It may be adorable, but enemies should not underestimate Umbreon.
Mega Absol
It may not be the best Mega Evolution in the series, but Mega Absol is a powerful Dark-type Pokémon. The biggest downside to using it is its limited access to attacks. Absol can learn some powerful Dark moves, but while Night Slash is available through leveling up, Dark Pulse is only accessible via TM. Even without this, though, Mega Absol is great offensively.
Its Speed and Special Attack are high, and its Attack is fantastic. Although it benefits most from Dark-type moves, it can use attacks of other types to devastating effect. Mega Absol cannot take many hits, but with its offensive power, it rarely has to.
Darkrai
The nightmare-inducing Darkrai is one of the most popular Generation IV Mythicals. It is not the strongest among them, but it has a gimmick that can make it deadly. Its Ability Bad Dreams reduces the HP of foes that are asleep. It has several moves that can put opponents to sleep and deal damage to them while they slumber. With great Speed, Darkrai can use its sleep-inducing actions before most opponents can strike. In addition, it has a high Special Attack stat, so it also excels with more conventional combat.