Bulbasaur might be the most accepting. It has a gentle nature. It could even seem to like the dress, perhaps sniffing at the fabric and then nuzzling Ash as if to show that it doesn't matter to it what Ash wears. It would still follow Ash's commands and be by his side as always.
Charizard could be a bit more indifferent. It's a proud and strong Pokémon. It might just snort when it sees Ash in a dress, as if to say 'This is a strange human thing, but it doesn't affect our relationship'. However, if other Pokémon made fun of Ash because of the dress, Charizard might get protective.
He might be initially shocked. Ash has always been surrounded by friends and supporters, so suddenly being betrayed by everyone would be a huge blow. He could be in disbelief for a while.
One possible plot twist could be that Ash wakes up one day and finds he has turned into a Pokémon himself. This would completely change his perspective and how he interacts with the Pokémon world.
One possible role is that he could be a spy. Since he was turned into a Pokémon, Team Rocket might not suspect his true identity and he could gather information about their evil plans. For example, he could overhear conversations between Team Rocket members that he couldn't have accessed as a human.
If Ash controls minds in the Pokémon fanfiction, it could lead to him being ostracized by other trainers. In the Pokémon world, such unethical behavior would not be tolerated. Other trainers might see him as a threat and try to stop him. This could create a lot of conflict in the story, with Ash having to either defend his actions or try to reverse the mind - control ability he has acquired.
One possible storyline could be Ash and his Greninja. They could go on a special mission to save a group of Pokémon from a natural disaster. Along the way, Greninja and Ash have to rely on each other's skills and instincts, strengthening their bond. Another could be Ash and his male Snorlax. Snorlax gets sick, and Ash has to find a rare herb to cure it, going through many challenges in the process.
One reason could be his sense of justice. Ash has always been about doing what's right in the Pokémon world, so saving all the legendaries would be a natural extension of that. Another reason might be his love for Pokémon in general. He wants to protect all Pokémon, and legendaries are no exception.
One possible plotline could be that Ash is forced into a dress as part of a prank by Team Rocket. They might use some sort of device to make him unable to take it off easily. Another could be in a different region where there's a strange cultural tradition that requires outsiders to wear a dress in a certain situation, and Ash, being unaware, gets caught up in it. Maybe his Pokémon try to help him get out of the situation in comical ways.
A Darkrai could be in Ash's secret team. Darkrai is known for its ability to create nightmares, which could be used to disorient opponents in battle. It's a very powerful and mysterious Pokémon that would add an element of surprise to Ash's team.
In fanfiction, Ash's secret Pokémon team could consist of some rare and powerful Pokémon. Maybe he has a shiny Charizard that he's been training in secret. It could have unique moves and a super strong bond with Ash, making it a formidable addition to his team.
On the other hand, it could also add some complications. Ash might face more jealousy or higher expectations from others. But overall, it would likely add a deeper layer of meaning to his journey. He could be on a quest not just to be a Pokémon master, but also to make his father proud. And he might be more eager to explore the mysteries of his family's connection to Pokémon, which could lead him to new and exciting adventures.