When it comes to Pokémon introduced in the Unova region, many are unpopular among fans. Some see them as uninspired or simply unlikable, but certain Pokes are quite strong nonetheless. The Legendary and Mythical Pocket Monsters from Generation V are especially capable in the main series Pokémon games.
The Unova region added nine new Legendaries and four new Mythicals, and while not all of them are exceptional, they are all relatively powerful in battle due to their classifications. Some creatures are only available late in the game or through specific events, but they can be viable at any point in a playthrough.
Virizion
The Grass/Fighting-type Legendary Virizion is in a bit of a strange position. It has a good Speed stat, but its offensive stats, while decent, are not high enough to capitalize on it. The upside to its Attack and Special Attack is that both share the same stat value, which can be advantageous if an opponent is vulnerable to either category of damage.
Virizion's Special Defense is great, but its Defense is lackluster. Its HP is decent and helps to make up for this a bit, but with six type weaknesses, one of which is an extreme vulnerability, Virizion can have difficulties in battle.
Cobalion
The Swords of Justice are Legendary Pokémon with some of the best lore in the series, but they are not the best of their classification. Cobalion has the same stat spread as Virizion, except for its Defense and Special Defense, which switch values. This gives Cobalion similar stat-related strengths and weaknesses.
However, Cobalion's typing gives it only three type weaknesses as opposed to Virizion's six, which helps to make up for its relatively poor Special Defense stat. It is still only decent in the offensive field, but it can usually stay in the battle a fair amount of time.
Keldeo
It may have the same Speed stat as Virizion and Cobalion, but Keldeo has the Special Attack stat to do some decent damage with it. The biggest problem is that through leveling up, it learns mostly Physical attacks, and Keldeo's Attack stat is underwhelming.
Its best Special Water move gained by leveling up is Hydro Pump. This move, while strong, is not always accurate. However, its signature move, Secret Sword, is a Special Fighting move, so it has a capable option in the Fighting department.
Terrakion
Though not the strongest Legendary in Generation V, Terrakion is the best among the Swords of Justice. Its stat spread mirrors that of Keldeo, but with the offensive stats switched. With this, Terrakion can properly utilize the excellent Physical attacks it can learn through leveling up.
Terrakion suffers from seven type weaknesses, which is unfortunate, but its capable bulk allows it to take decent damage. If it battles a foe that heavily resists Physical damage, though, it can run into some problems.
Victini
Victini continues the trend set by Mew for certain Mythicals to have an exactly even stat spread. As a result, it is bulky but not exceptionally so, and it is offensively strong but not a powerhouse. It has plenty of great move options, and its signature Ability Victory Star slightly increases its accuracy and that of its allies.
Not having a specialty can make Victini a less desirable pick if trainers want a higher damage output, but its stat spread also provides it with consistency in withstanding and dealing decent damage.
Meloetta
With the ability to transform between variations, Meloetta can change its secondary typing and stat spread, which is part of what makes it one of the best dual-type Normal Pokémon.
When in its Aria Forme, it focuses on its Special stats, but its Physical stats suffer a bit. When in its Pirouette Forme, its Attack and Speed gain a massive boost, and its Defense raises slightly as well. This is at the sacrifice of its Special stats. Trainers can use this ability to create better matchups against a variety of opponents.
Thundurus
Two form options give Thundurus different stats, although the change is not as dramatic as Meloetta's. In its Incarnate Forme, it has fairly even offensive stats, both of which are good but not great. In its Therian Forme, it focuses more on its Special Attack, making this stat much higher. In this variation, it has the Ability Volt Absorb, which negates Electric damage against it and restores some of its health instead.
In its Incarnate Forme, it can have the Hidden Ability Defiant, which raises its Attack when an opponent lowers its stats. In the end, both Thundurus variations have only decent bulk, which is part of why it is not as strong as many other Legendaries.
Tornadus
Tornadus's Incarnate Forme gives it more offensive power, while its Therian Forme makes its stats a bit more even, also providing it with a slightly higher Speed stat.
In its Therian Forme, Tornadus also has the Ability Regenerator, which restores part of its health when it switches out of battle. This helps because while Tornadus's bulk is better than that of Thundurus, it is still only decent. On the other hand, when Tornadus is in its Therian Forme, its offensive stats become unimpressive by Legendary standards.
Landorus
With better bulk than both Thundurus and Tornadus, Landorus can withstand more attacks, as long as they are not Ice-type, to which it has an extreme vulnerability. The offensive stats of its Incarnate Forme are more even, while those of its Therian Forme favor the Physical side.
As with the other members of the Forces of Nature, the form change can be beneficial or detrimental depending on the matchup. Both forms of Landorus are decently fast but not exceptionally so, but its bulk is usually enough to let it deal at least a couple of strikes.
Genesect
With the typing of Bug/Steel, one of the best dual-type combinations in Pokémon, Genesect partially makes up for its decent but not fantastic bulk, resisting eight types. Offensively, Genesect has fairly high stat values in both categories, and its Ability Download is excellent.
This Ability boosts Genesect's Attack or Special Attack depending on which of the opponent's defensive stats is lower. In addition, the type of its move Techno Blast changes depending on the Drive that Genesect holds, which can be helpful if trainers know the coming matchup.
Kyurem
Kyurem has two more powerful forms in Black Kyurem and White Kyurem, but these require combining with Zekrom and Reshiram, respectively. On its own, though, Kyurem is still exceptional. Both its offensive stats have the same high value, and while its defensive stats are not incredible, its great HP more than makes up for it.
Kyurem has five weaknesses, which can be difficult, but even so, it can usually take a few hits. This powerful creature is a significant part of the lore of the Original Dragon, and Kyurem would be a great candidate for a future game in the series like Legends: Arceus.
Zekrom
The Dragon/Electric Zekrom lives up to its status as a box art Legendary. Its Attack stat is fantastic, allowing it to effectively use the excellent Physical moves it can access through leveling up. Its Special Attack, while lower than its Attack, is still fairly high, so it can deal with opponents that defensively specialize in either category.
It also has great bulk, which compensates for its only decent Speed. Though it initially only had three type weaknesses, it gained a fourth in Generation VI with the introduction of the Fairy-type. But, Zekrom can still dominate in battle.
Reshiram
The Dragon/Fire Reshiram has the same stat spread as Zekrom, but the Special and Physical stats switch values. As such, Reshiram excels as a Special attacker, and its excellent move pool reflects it. It has the same weakness in its Speed, but like Zekrom, its bulk makes up for it.
One of the main reasons why Reshiram is more viable than Zekrom in the main series games is that while Zekrom gained a Fairy weakness in Generation VI, Reshiram did not, leaving it with only three type weaknesses. It deserves its place as a box art Legendary.