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.
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.
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.
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.
One common theme is the exploration of their complex relationship. Given their different personalities and playing styles on the court, fanfictions often delve into how they understand and support each other off the court. For example, how Akashi's strict and calculating nature might be softened by Kuroko's quiet and observant presence.
One interesting Kagami and Kuroko lemon fanfiction could be the one where they go on a trip together and their relationship deepens in a really sweet and passionate way. It might include some tender moments between them that are unique to their characters.