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.
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.
Pikachu is a very likely candidate. It's Ash's most iconic Pokemon and has a deep bond with him. Whenever Ash is in trouble, Pikachu often steps up to protect him.
A possible plotline could be that Ash is in a tournament and all his regular Pokémon are unable to compete further. So, he calls upon the strongest Pokémon, say Lugia. Lugia then uses its amazing powers to defeat all the opponents and Ash wins the tournament, which also earns him respect from all the trainers.
Ash is that energetic trainer we all know from Pokemon, always eager to take on new challenges and befriend more Pokemon. Astrid in Pokemon fanfiction can be seen as a fresh addition to Ash's world. She may be designed to add new dynamics to the story. Maybe she has a different approach to Pokemon training compared to Ash, like focusing more on Pokemon's natural instincts rather than just battle strategies. And she and Ash could start off not getting along but then gradually become good friends as they share experiences in the Pokemon world.
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 could also have Charizard. It's a powerful and well - loved Pokemon of his. It could provide a lot of firepower in battles. For Gray, a Glaceon would be great. Glaceon's ice - type moves could complement Gray's more strategic style. It could freeze opponents and give Gray time to plan his next move.
Maybe it's a legendary Pokemon in disguise. In some fanfictions, legendary Pokemon often play important roles. It could give Ash the egg as a test or because it senses that Ash has a pure heart. Another possibility is that it could be a young trainer who can't take care of the egg anymore and trusts Ash to do a good job.