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Ash is the very best, like no one ever was, but it was a long road to get there. At the start of his journey, Ash was an inexperienced trainer, far from becoming one of the greatesttrainers in Pokémon. By the end of the Masters Eight Tournament, Ash had become a skilled, strategic trainer whose strong bond with his Pokémon allowed him to pull off feats few other trainers could.
While Ash doesn't always have the best knack for team composition, he's caught some extremely powerful Pokémon on his journeys who helped carry him through the different regions he visited. The strongest Pokémon in every region Ash visited carried their weight and won him key matches no other Pokémon could. Ash technically does go to Galar for the Masters Eight Tournament, but doesn't explore the region like in earlier seasons, so he has no Galarian ace.
Kanto: Pikachu
Although Charizard acted as Ash's ace back in the original series, by the end of his career, Pikachu was clearly Ash's ace. Ash's Pikachu has pulled off some impressive feats that would be impossible in the games, like beating Brock's Onix with a Thunderbolt. Pikachu is freakishly strong and fast, capable of taking down some extremely powerful opponents.
There is no Pokémon Ash trusts more than Pikachu, and their bond only makes Pikachu stronger. Pikachu defeated Leon's Charizard, one of the strongest Pokémon in the entire franchise, and also beat multiple Legendary Pokémon, including Tapu Koko and Regice. Pikachu also Volt Tackles his way through a Luster Cannon to knock out a Latios in one of its coolest feats in the series. Pikachu isn't just Ash's Kanto ace, he's his whole team's ace. Ash couldn't have won the Masters Eight without Pikachu.
Johto: Heracross
Ash didn't really have an ace while he was in Johto. He regularly rotated between Pikachu, Cyndaquil, and Bayleef for most of his battles, but Heracross stands out thanks to some impressive feats. Heracross goes on to be a major player in Johto's Silver Conference, allowing Ash to score against Gary and settle their rivalry once and for all.
Heracross is a quick, powerful Pokémon that can stack up against odds just like Pikachu. Heracross straight up tanks a Flamethrower from Gary's Magmar and still has more than enough energy to keep fighting. Ash's Heracross is a reliable, sturdy Pokémon that can pull its weight when push comes to shove.
Hoenn: Sceptile
Sceptile is one of the strongest Pokémon Ash has ever had on his team. Treecko was already strong for a starter, before really making a name for itself as a Grovyle. Ash's Grovyle was far and away his best Pokémon during the Hoenn League, pulling off some major wins, like beating Katie's Walrein in an incredible battle. Leaf Blade is downright deadly in Grovyle/Sceptile's hands.
Grovyle finally evolves into Sceptile during the Sinnoh League, earning Ash a well-earned pyrrhic victory when Sceptile defeats Tobias' Darkrai in one of the anime's best fights. Ash did not end up using Sceptile in the Masters Eight Tournament, but that doesn't diminish its accomplishment. Defeating a Legendary is more than most Pokémon can lay claim to.
Sinnoh: Infernape
Infernape is one of the best starters Ash has ever raised in the Pokémon anime. Infernape originally belonged to Paul as a Chimchar, before he abandoned him and Ash brought him onto his team. This ends up being a huge mistake on Paul's part, as Chimchar goes on to become extremely powerful. During Ash's journey throughout Sinnoh, he uses Infernape often.
Not only that, Infernape actually wins most of his fights, boasting one of the best battle records of all of Ash's Pokémon, at an impressive 77% of fights won. Infernape's best fight is his battle against Paul's Electivire in the Lily of the Valley Conference, where he defeats Paul's new ace in expert fashion.
Unova: Krookodile
Krokorok was a true staple of Ash's team in Unova. While Ash regressed as a trainer in the Pokémon anime's adaptation of Black & White, Krokorok proved that Ash wasn't totally hopeless. Even before evolving into Krookodile proved itself to be a strong and reliable Pokémon (albeit weaker than Pikachu).
While battling Iris in the Pokémon World Tournament Junior Cup, Krokorok finally evolves into Krookodile. Krookodile is powerful enough to take out of her Dragonite. Krookodile also gets an amazing showing when Ash later battles Stephan. Unova may have been a step back for Ash, but Krookodile really shined as one of the best Pokémon.
Kalos: Greninja
Ash's bond with Greninja is truly amazing, and would rival his relationship with Pikachu if it weren't foundational to the entire anime. Greninja is one of the strongest Pokémon on Ash's team, and has some truly legendary feats. Greninja manages to resist his type-disadvantage against Sawyer's Mega Sceptile, ultimately winning in the end. Greninja also puts up an amazing fight against Diantha's Mega Gardevoir, Kalos' Champion.
Greninja becomes even stronger when turning into Ash-Greninja, a transformation born out of Ash and Greninja's strong connection. Ash releases Greninja in Kalos, so he does not use him during the Masters Eight Tournament, but it's not like Greninja was in need of more time in the spotlight. Greninja has an amazing showing throughout Kalos, participating in numerous high-stakes battles, all while boasting a respectable win record.
Alola: Lycanroc
Ash never would have won his first ever Pokémon League if not for Lycanroc. Lycanroc was easily Ash's ace Pokémon during his time in Alola. While Incineroar may be stronger, Lycanroc's achievements and feats are far more impactful to Ash's journey as a trainer. Ash's Lycanroc ends up battling and beating Gladion's Lycanroc in the final battle of the Manalo Conference.
Lycanroc also played a key role in landing Ash the win against Nanu and Olivia. Lycanroc participates in some important battles in Alola, developing the skill set necessary to win Ash the League in the end. Like Johto, Ash rotates his Pokémon fairly often in Alola, but Lycanroc is clearly the region's ace.
