One possible strongest team could be Pikachu, Charizard, Greninja, Lucario, Sceptile, and Infernape. Pikachu has been with Ash since the start and has a strong bond. Charizard is extremely powerful. Greninja has its unique form and great combat skills. Lucario is a very capable fighter. Sceptile was a strong addition to Ash's team in Hoenn. And Infernape showed great power in Sinnoh.
In fanfiction, Ash's strongest team might also consist of Dragonite, Gengar, Snorlax, Blastoise, Alakazam, and Tyranitar. Dragonite is a very powerful and fast dragon - type. Gengar can be tricky and has strong attacking moves. Snorlax is a bulky powerhouse. Blastoise has great water - based attacks. Alakazam is highly intelligent and has strong psychic powers. Tyranitar is a force to be reckoned with.
For Ash, Pikachu is always a great choice as it's his most loyal and powerful partner. Alongside that, a strong flying type like Charizard could be useful. For Brock, his Onix would be a staple. And perhaps a water - type like Vaporeon could be good for him to balance his team. Since they are teaming up, having a psychic - type like Alakazam could also be beneficial as it can help in battles and also with communication in tricky situations.
Ash has always been a defender of Pokemon. If he found out about Team Plasma's experiments, he would first try to understand the full scope of what they were doing. Then, he would use his experience and his Pokemon team's skills to fight against Team Plasma. He might also try to convince some of the members of Team Plasma that what they are doing is wrong, as he has a way of seeing the good in people. Ash would be very determined and would not give up until he put an end to the experiments.
He might become more cautious. Usually, Ash is quite bold and rushes into things. But if he's in a Team Rocket - centered fanfiction, he'd likely be more careful as he's constantly surrounded by danger. For example, he'd think twice before challenging a new Pokemon or entering an unknown area.
If Team Rocket turns Ash into a Pokemon, it could lead to a really interesting story. Ash would have to experience the world from a Pokemon's perspective. He'd understand their struggles, like being caught and trained. This could also be a way for him to better understand his own Pokemon and their bond with him. And it would be a wild adventure as he tries to find a way to turn back to his human form.
In a fanfiction where Ash joins Team Rocket, it could lead to some really interesting storylines. Ash has a lot of knowledge about Pokémon training that Team Rocket lacks. He could potentially teach them new techniques, but at the same time, his moral compass would be constantly at odds with their devious ways. Maybe he joins them because he's somehow tricked or he has his own secret plan to take them down from the inside. It would be a great opportunity to explore different aspects of his character and how he adapts to this new situation.
One possibility is that Ash joins Team Galactic under some sort of misunderstanding. Maybe he thinks they are a group with good intentions at first. Once he realizes their true nature, he has to figure out a way to escape or stop them. This could create a lot of tension and action - packed scenes in the fanfiction.
He could have a Froakie. It's a common and cute starter in Kalos. Maybe also a Skiddo for traveling across the Kalos landscapes. And a Bunnelby would be a nice addition for its digging abilities.
One of the popular fanfics where Ash calls the strongest Pokémon could be those where he summons Arceus. In some fanfictions, Ash might have a special connection with Arceus, the so - called 'God of Pokémon'. This could lead to epic battles and adventures as he teams up with the most powerful being in the Pokémon universe.
Well, if Ash starts with an Ekans, his team dynamic would be very different. Ekans is a snake - like Pokemon, and it might not get along well with some of the more energetic or flighty Pokemon at first. For instance, Pikachu might be a bit wary of it since they are so different in nature.
It would likely bring a new level of harmony (literally) to Ash's team. Since Meloetta has musical powers, it could use its melodies to boost the morale of the other Pokémon on the team. For example, before a big battle, Meloetta could sing a song that makes Pikachu feel more energetic and ready to fight.