Ash Ketchum's dream has always been to become a Pokémon Master and in doing that, he wanted to catch all of the Pokémon out there. While he's come nowhere close to accomplishing that, despite traveling to various regions, he's still amassed an impressive collection.
But no matter how many Pokémon Ash has at his disposal, he only carries five or six with him at a time and has parted ways with countless partners -- either by leaving them with Professor Oak, sending them to train elsewhere, or releasing them into the wild. Let's take a look at five Pokémon that Ash should have kept with him.
Charizard
Pikachu has always been Ash's main Pokémon companion and overall fan-favorite, but Charizard is not too far behind. Their connection goes all the way back to the Kanto region when Ash saved a Charmander who was abandoned by his trainer. Ash trained Charmander until he evolved into Charmeleon and then Charizard. Charizard wasn't an easy Pokémon for Ash to control due to hardly ever listening to commands. Ash made the difficult decision to leave Charizard with Liza at Charicific Valley to train. However, Charizard was reunited was Ash in the Unova region. After that journey concluded, Ash left Charizard with Professor Oak.
Charizard may be one of the most challenging Pokémon Ash has trained, but he's also one of the strongest. He's had unforgettable battles with the likes of Blaine's Magmar and Clair's Dragonair and has proved to be a great asset to Ash's team. While letting him go to train was a good decision, Ash should have considered keeping Charizard around after his skills improved and he became even more of a force to be reckoned with.
Greninja
Ash always tries to build a bond with any Pokémon he catches, but his bond with Greninja is a little different. When Greninja was just a Froakie, he was a loner, and selective about his trainers due to high expectations. However, after assisting Ash and Pikachu in a battle against Team Rocket and being helped back to Professor Sycamore's lab by Ash, Froakie asked Ash to capture him.
This built a strong relationship between the two that continued to grow as Froakie evolved into Frogadier and then Greninja. The two trained so hard and developed such a bond together that Greninja was even able to achieve a form called "Ash-Greninja," in which he took on an appearance similar to that of his trainer. Greninja is the only Pokémon of Ash's who can achieve such a feat. It's also an extremely powerful form that has come in handy in many different battles.
Still, after a long journey together, Ash and Greninja parted ways when Team Flare caused powerful roots to emerge and Greninja went to help destroy them. While this was a noble cause, Greninja had proved himself to be a valuable and unique member of Ash's team. It's unlikely that Ash will connect with another Pokémon the same way he did with Greninja, so it couldn't have been an easy decision to let him go. Maybe when Greninja's work is done or if Ash ever returns to the Kalos region, the two will have a touching reunion.
Butterfree
Butterfree was the first Pokémon Ash said goodbye to on his journey and fans still remember that emotional goodbye to this day. Ash caught Butterfree in Veridian Forest when he was just a Caterpie -- the first Pokémon Ash ever caught in the wild. When he evolved into Metapod, he engaged in the ultimate hardening battle with another Metapod, a battle that was interrupted by a swarm of Beedrill, forcing Ash to leave Metapod behind. He owned up to his mistake, which led to Metapod evolving into Butterfree and stopped the Beedrill swarm with Sleep Powder. Ash didn't have Butterfree for too long, though. During a Butterfree mating ritual, Ash's Butterfree found a mate and Ash decided it would be best for him to go with her. Since then, he's never seen Butterfree again.
While it was nice of Ash to think about Butterfree's feelings, Butterfree could be a useful part of his team. Ash hasn't really had a go-to Bug-type and since Butterfree is a Bug and Flying dual-type, as well as being able to learn powerful moves like Psychic, he could have been and still could be useful in countless battles.
Primeape
Speaking of Pokémon Ash didn't have on his team for long, Primeape only traveled with him for four episodes. Ash met Primeape when he was a Mankey and stole Ash's hat. He evolved into Primeape during a battle with Team Rocket and Ash caught him during that battle as well. Ash never relied on Primeape in battle due to his aggressive nature. When they were in Celadon City, Ash trained Primeape to win a Fighting-type tournament called the P1 Grand Prix. Afterward, he left Primeape with a Fighting-type trainer named Anthony for extra training and Primeape hasn't been seen since.
Primeape may have been aggressive, but he was warming up to Ash toward the end of their journey together. With some more bonding time between the two, Primeape could have been one of Ash's more powerful Pokémon during his Kanto journeys and maybe even beyond.
Goodra
Goodra is another Pokémon that Ash released back into the wild. Ash caught him in the wetlands when he was Goomy. After a long journey and a lot of hard-fought battles in the Kalos region, Ash released Goodra back to the wetlands, causing everything to come full circle.
With Goodra being a pseudo-Legendary, he was one of the strongest Pokémon on Ash's team at the time. He's a Dragon-type with a powerful offense and could have been useful to Ash in a lot more battles if given the chance. If there's ever a reunion between these two, hopefully, Goodra can be seen battling at full force once again.