During Ash's time as the protagonist of Pokémon anime, fans were introduced to dozens of creatures who would become important and beloved members of his various teams. Considering the boy is by far one of the strongest trainers in the series, it's no surprise that he was able to instruct his partners to become powerful and impressive fighters. However, despite Ash's innate talent as a trainer, he was also blessed with the luck of often finding Pokemon whose power was above average.
On several occasions, the protagonist's trusted monsters displayed levels of strength and resilience that rivaled the strongest Pokémon in the entire franchise. Ash would also often encounter creatures with hidden and unique abilities that make them several times more powerful than other members of their species. While examples of this phenomenon are not few, only a handful, among the protagonist's strongest Pokémon ever, can be seen as truly overpowered.

Pokemon fans have decoded Pikachu's “Pika-Speak” language
Pikachu's limited vocabulary of repeating syllables can actually be a sophisticated form of communication, with fans noticing certain patterns.
10
Gible
Like most people Pokémon fans are aware that the first stage evolutions tend to be much weaker than their final version, even when they are part of the line of one of the strongest pseudo-legendaries ever, like Gible. As such, when fans were introduced to this adorable and wild creature, they expected to one day see it evolve into Garchomp, who could have become one of Ash's strongest partners ever. What no one could have seen coming was Gible training to become the most prominent user of the Draco Meteor in the world.
Despite being only a small and somewhat weak Pokémon, Ash's little friend could fire powerful attacks that could knock out even fully developed opponents. The level of power exhibited by this monster was unlike anything that had been seen by a First Evolution in the past. Gible more than earned his spot as one of Ash's best Dragon-Types, despite his young age and low stats.
9
Incinerate
Despite being one of the most skilled coaches in Pokémon anime, Ash was never one to rely on his experience and strength alone. In order to never become stagnant, he often sought help from other powerful combatants, as was the case in the meantime Sun and moon era of anime. After acquiring a powerful but inexperienced Torracat. Kukui, one of the many awesome professors in the franchise, offered to help train the cat-like monster, as he had a powerful Incineroar.
Ash's Fire-Type friend learned a lot during his training and eventually became so powerful that he defeated his mentor in battle, even before it had fully evolved. This victory gave Torracat enough battle experience to finally unlock his final transformation and become a mighty Incineroar. Regrettably, this version of the Pokémon was never used by Ash in battle, although it can be inferred from its past that it would have been much stronger than other members of its species.
8
Gangs
Arguably one of the characters with the most tragic backstories and a fundamental part of Ash's stronger team ever, Gengar was a trickster with a heart of gold and a natural talent for combat. Joining the protagonist after his original trainer was abandoned, this adorable Ghost-Type worked hard to become a reliable teammate, and it more than achieved its goal. Its attacks were much more effective and powerful than those of a regular Gengar, greatly aiding Ash in battle.
But what really helped this Pokemon stand out from the rest is the fact that it had the elusive Gigantamax factor. Thanks to this, and the fact that Gengar is one of many species that can access a Gigantamax form, Ash was able to use this ability to greatly increase his friend's power. With this power boost, the mischievous hero was able to defeat several strong opponents, such as Marnie's Grimmsnarl.
7
Krookodile
Due to the fact that they are almost always the deciding factor in battle, it is no secret that Pokémon fans that the types and their strengths are among the most important aspects of the series. Even if a trainer's creature has more power or is more resilient than its opponent, a bad spell matchup can result in defeat. While this is true of almost every monster in the anime, Ash's Krokodile seems to be able to ignore almost any type of weakness, as evidenced by his impeccable record.

Pokemon fan uses 3D printer to make cheeky Gengar Lich King
A Pokemon fan uses a 3D printer to create a Gengar wearing the helm and mantle of The Lich King, one of World of Warcraft's iconic antagonists.
One of its greatest feats comes from Ash's battle against Brycen, one of Unova's strongest trainers, who specializes in Ice-Type Pokémon, Krookodile's direct counterpart. The crocodile-like monster not only managed to withstand multiple attacks that were super effective against it, but it also defeated some of the opponent's most powerful partners. Being able to survive continuous Ice-Type attacks, when a single one should have been able to take it down, makes Krookodile a unique and rare member of its species.
6
Charizard
By far one of the best and most popular Pokémon among the original 151, as well as one of Ash's oldest friends, Charizard's story is one of friendship and resilience. During the original Indigo League series, considered by many to be one of the show's best seasons, this iconic creature was known for his disrespect and defiance towards his trainer. It took time and special training in the Charicific Valley to make him a reliable and impressive partner.
Charizard's Fire-Type attacks were already quite powerful, probably in the highest proportion of its species, but it really shined thanks to its impressive physical strength. The grueling training it underwent allowed it to fight toe to toe against opponents that should have easily defeated Ash's friend. Both Gary's Blastoise and Nolan's Articuno were defeated thanks to Charizard's devastating Seismic Toss. That's why it's not surprising that this beloved character is among Ash's best Pokemon in terms of win rate.
5
Lucario
For more than a decade, since its first appearance back under Diamond and pearl era of the franchise, Lucario has been one of the most popular Pokémon of all time. It's easy to see why, as not only does it have one of the best designs in the series, but it's also an extremely useful team member. When Ash was able to acquire a Riolu, sensing the strong aura from the boy, fans expected it to become one of the protagonist's aces in no time.
As expected, that's exactly what happened, with Lucario developing into a vital asset to Ash's team during Tripera of anime. Its usefulness grew to the point that it became the second Pokémon to bond with the protagonist to such a level that it unlocked a new level of Aura control. Lucario's movements also became linked to Ash's, giving him a boost in agility and reaction time. Although this band is probably not unique, as a fan favorite Pokémon the movie proves that it was quite impressive.
4
Infernape
Those who grew up under Diamond and pearl the era of Pokémon series will remember Ash's Chimchar, one of the creatures with the best character arcs and impressive growth. Originally a member of Paul's team, the ape-like monster was abandoned because it was considered weak and useless. Ash, who can see the best in all Pokemon, took him as part of his team and trained him to be a real threat in battle.
After evolving into Infernape, its meek personality was replaced with a fierce desire to prove its worth and a physical strength that could compete with low-level legendary Pokémon. This creature could defeat opponents quickly with accurate and destructive attacks. If the initial attack wasn't enough, Infernape also had the Blaze ability, which increased his power the more severe the damage took, easily making it one of the best Fire-Types ever.
3
Sceptile
While those who have dug into the world of Pokémon Video games know that Sceptile is one of the worst starters, Ash's partner rose above this stereotype and became an honorable and fierce warrior. Trained since it was a Treeko, this creature stood out due to its overwhelming strength and its ability to surprise opponents.
Although its physical power was not outstanding among Ash's other Pokemon, its speed and battle intellect were far above most others. Sceptile often tried to use his speed to wear down opponents, avoid attacks, and deliver precise hits that would slowly wear the opponent down. This strategy was enough to defeat the legendary Pokémon Darkrai, wielded by one of Ash's strongest opponents ever, Tobias.
2
Greninja
Of the plethora of water-type Pokémon Ash caught throughout its journey, Greninja is objectively the best and most memorable. At first, this monster was a brave but inexperienced Froakie, who joined the main character soon after he arrived in the Kalos region. Its training was hard, as it had a deep desire to become a reliable partner for Ash, a goal that Greninja more than achieved.

Fan designs regional evolutions for Random Starter Pokemon
A Pokemon fan shares his impressive regional redesigns of three starter Pokemon from multiple generations of games.
Not only did this amazing creature evolve into a fast and efficient fighter that would use its impressive attacks and speed to dominate its opponents, but it also became the first of its kind to unlock a new form. Its deep bond with Ash transformed it into the very popular Ash Greninja, one of the strongest Pokémon ever in the series. Thanks to this ability, the already powerful monster was able to easily reach the level of the most powerful legendaries, despite the plot forcing it to lose during the final battle of the X and Y era.
1
Pikachu
The Pokémon anime has introduced a large list of beloved characters during its more than two decades on air, but none have become as iconic to the series as Ash's first and oldest friend: Pikachu. Once an unruly and arrogant creature that refused to listen to its trainer, this adorable monster grew into one of the most loving and powerful Pokémon of all time.
Despite never evolving, Pikachu quickly became the only partner worthy of being Ash's strongest teammate ever. Due to being a part of almost every major battle the trainer participated in, the franchise's signature Pokémon grew to a level never seen before. Pikachu is capable of challenging the most powerful gods in the world of the series, and it would have more than a reasonable chance of winning.