It's no secret that Pokemon certainly aren't created equal, and some creatures just have more power by default. This fact is not dependent on whether a species has one elemental type or two. However, many dual-type Pocket Monsters are among the most powerful in the entire series' history. Put plainly, trainers would be remiss not to seek these creatures when they have the chance.
Although a Pokemon's power in the in-universe lore is one thing, it may be better to measure their strength by their base stat total. This indicates the Pocket Monsters who have been given the most strength across their stat spread before they've even begun to be EV trained to grow stronger.
As of Pokemon's Generation IX games, the top contenders have remained mostly locked in place with very little movement among the strongest dual-types.
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Most powerful dual-type Pokemon based on maximum base stats
1) Eternamax Eternatus
The Poison/Dragon-type Eternatus was already a formidable Pokemon to begin with, but its base stats go into overdrive when it converts into its Eternamax form in the Sword and Shield games. This supersized form gives Eternatus a maximum base stat total of 1125 points, which is much higher than even the best dual-types in the series.
Eternamax Eternatus' base stat pool allows it to have immensely high HP, defense, and special defense without any training needed. This makes the Galar region's antagonistic Legendary Pokemon a hulking tank of a creature that can be tough to take down.
2) Mega Mewtwo X
Mega Mewtwo first arrived in Pokemon X and Y, and it hasn't fallen from the top dual-type charts ever since. Despite the mono Psychic-type Mega Mewtwo Y sharing its base stat total of 780 points, Mega Mewtwo X is a dual Fighting/Psychic-type, giving it extra versatility on offense.
Mega Mewtwo X benefits from having exceptionally high base stats for both its Attack and Special Attack, making it an offensive powerhouse capable of dealing serious damage with both physical and special moves.
3) Mega Rayquaza
Rayquaza was already an absolute beast when it arrived in the Generation III Pokemon games, but its Mega Evolution introduced in the Gen III remakes Omega Ruby and Alpha Sapphire skyrocketed its power. Overall, Mega Rayquaza matches Mega Mewtwo X's base stats with a total of 780 points.
With 180 points in both its Attack and Special Attack stats, Mega Rayquaza is more than a match for nearly any opponent it faces. Coupled with its Delta Stream ability, which removes its Flying-type weaknesses, it's no secret that this Dragon/Flying-type Pokemon is one of the strongest ever created.
4) Primal Groudon
The Generation III legendary Groudon was already a force to be reckoned with, but the advent of Omega Ruby and Alpha Sapphire saw its power magnify. When holding the Red Orb, Groudon undergoes Primal Reversion and becomes Primal Groudon, a Fire/Ground-type species.
In this form, it sports a base stat total of 770, one it shares with Primal Kyogre. However, Primal Kyogre is a mono Water-type, so it can't quite qualify for this list the way that Primal Groudon can. Even better, Primal Groudon's ability Desolate Land nullifies Water-type attacks (its biggest weakness) by changing the weather.
5) Ultra Necrozma
Necrozma is a Pocket Monster of many forms, but it reaches its true potential while in its Dusk Mane or Dawn Wings form and holding the Ultranecrozium Z item while using the attack Ultra Burst. This transforms the Generation VII legendary into Ultra Necrozma, a Psychic/Dragon-type with immense power.
In its Ultra Form, Necrozma has a maximum base stat total of 754 points. These points are mainly concentrated in its Attack, Special Attack, and Speed stats, making Ultra Necrozma incredibly quick and deadly. If that wasn't enough, its Neuroforce ability increases the damage of its super-effective attacks.