It's hard to name the 'best' as it's subjective. But some popular writers might be those who have a deep understanding of the characters. Writers who can accurately portray Aomine's laid - back yet intense nature and Kagami's hot - headed but determined personality tend to create engaging fanfictions.
One interesting plot could be Aomine and Kagami having to team up against a common, extremely powerful opponent. They start off bickering as usual but gradually learn to trust and complement each other's skills.
Well, Kuroko and a kind - hearted character from Kasami Kiss could make an interesting pairing. Kuroko's quiet and observant nature could contrast well with the more outgoing and emotional nature of a Kasami Kiss character. They could learn a lot from each other.
Perhaps a story where a character from Kuroko no Basket has to teach a character from Kasami Kiss how to play basketball. This could lead to a lot of comical moments as the Kasami Kiss character tries to understand the rules and techniques, while also developing a friendship or something more with the Kuroko no Basket character.
You can try searching on fanfiction websites like Archive of Our Own (AO3). There are often a lot of fanfictions about Kuroko and Kagami there. Just use the search bar and type in their names to find relevant lemon fanfictions.
Growth is also a big theme. Fans like to write about how Kuroko and Kagami grow as players and as people. They start from a certain point and through various experiences, they become better versions of themselves. It could be in terms of their basketball techniques or their ability to handle different situations.
One characteristic is the exploration of their contrasting personalities. Akashi is often portrayed as strong, confident and a natural leader, while Kuroko is more introverted and underestimated. Fanfics usually play with how these two personalities interact and complement each other.
One great Akashi x Kuroko fanfiction is 'The Red and the Blue'. It beautifully delves into their relationship, showing how Akashi's dominant personality and Kuroko's calm nature complement each other. The author does an excellent job of portraying their emotions in a Yaoi context.
I really like 'Kuroko's New Adventure'. In this fanfic, Kuroko discovers a new basketball technique that challenges him and his team. It's full of action - packed basketball scenes and also delves into the relationships between the characters, adding more depth to their bonds.
One great 'Kuroko no Basket Kuroko x Momoi fanfic' is 'A Chance Encounter'. It beautifully portrays their relationship in a different light, with a focus on how they support each other outside of the basketball court. Another interesting one is 'Unspoken Bonds' which delves into their unspoken feelings and how they gradually come to terms with them.