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.
There are also some fanfics that explore more of a mentor - mentee relationship between them. Maybe Kuroko helps Murasakibara in some aspect of his life or basketball skills, or vice versa. This kind of relationship can create really heartwarming and inspiring stories.
Growth is also a major theme. Both characters are shown to grow, either in terms of their basketball skills or in their personal relationships. For example, Murasakibara might become more motivated, and Kuroko might become more assertive in some fanfics. And these changes are often related to their relationship with each other.
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.