Articuno, Zapdos, and Moltres are quite common. Their elemental powers and the mystery surrounding them make for great fanfiction material. Ash might encounter them during his travels and have to either help them or face off against them in some way.
Lugia and Ho - Oh are also often featured. Their majestic appearance and their significance in the pokemon lore make them popular choices. In fanfiction, Ash could be involved in a quest related to these two, like protecting their habitats or helping them hatch their eggs.
Arceus is quite popular. Given that it's considered the creator of the Pokemon universe in the official lore, it often has a significant role in fanfictions. Mewtwo is also very common. Its powerful psychic abilities and complex backstory make it a favorite for fanfic writers to explore. Another popular one is Lugia. With its majestic appearance and its connection to the sea, it's often a central figure in many stories.
One of the Legendary Pokemon could be Giratina. Despite its somewhat dark and otherworldly nature, Giratina might be intrigued by Ash's light - heartedness and his ability to make friends with all kinds of Pokemon. Dialga and Palkia could also be part of the harem. Dialga, with its control over time, might see Ash as an important factor in the future of the Pokemon world. Palkia, in charge of space, could be interested in how Ash navigates different regions and spaces in his journey. And don't forget about Ho - Oh. Ho - Oh is a symbol of hope and might see Ash as a bringer of hope in the Pokemon world, thus joining his harem.
Obviously, Ash is one of the main characters. Since it's about him being a legendary Pokemon in this fanfiction, he's central to the story.
There are also fanfictions where Ash catches Rayquaza. This could be set during a time when the sky is in chaos due to some dark energy. Rayquaza is drawn to Ash because of his pure heart and determination. Ash, with his usual courage, teams up with his pokemon to assist Rayquaza in restoring balance. As a result, Rayquaza allows Ash to catch it.
The appeal lies in seeing Ash in new and exciting situations. With legendaries involved, his journey becomes more epic. It's like taking the underdog story of Ash in the anime and supercharging it.
In Ash-centered legendary Pokemon fanfiction, often Ash has extraordinary adventures. He might form unique bonds with legendary Pokemon that are not seen in the official storylines. For example, he could befriend a Mewtwo and together they face off against an evil organization trying to clone more powerful Pokemon. These fanfictions allow for a lot of creativity, expanding on Ash's character and his relationships in the Pokemon world.
Mewtwo is also very popular. Given its origin as a genetically - engineered Pokemon, a human could interact with it in many ways. Maybe a human scientist wants to study Mewtwo further, or a young trainer tries to befriend Mewtwo and help it come to terms with its existence and powers.
Obviously, Ash is one of the main characters. He's the protagonist who has a passion for pokemon and is always eager to become a pokemon master. And the legendary pokemon he encounters or befriends would also be important characters. For instance, if it's Lugia, Lugia would play a major role in the story as it's a very powerful and mysterious pokemon.
One of the popular pairings is Ash and Pikachu with a Legendary Pokemon like Mewtwo. They could form an unlikely alliance to fight against a common enemy. Another could be Ash and a female trainer along with a Legendary Pokemon, like Ash, Misty and Lugia. Their combined skills and the power of the Legendary Pokemon make for an interesting dynamic.
I think Salamence would be quite challenging. It's a very powerful and aggressive Pokemon. Ash would need to have a really strong team to weaken it first. Also, its flying ability gives it an advantage in battle, making it hard for Ash to land a final blow for the catch.