It's tough to pick a single 'best' because it really depends on what kind of dynamic you're looking for. Some people crave the pure, intense rivalry angle, while others want the domestic fluff.
For the classic rivals-to-lovers slow burn, 'Inertia' by Atrophaneura is practically required reading in the fandom. It handles the push-pull of their partnership post-high school with so much care, exploring what happens when the Seirin-era adrenaline fades and they're left with just each other. The basketball scenes are sharp, but the emotional tension in the quiet moments is what really sticks with you.
If you prefer something that leans into the supernatural hints of the series, 'Phantom Limb' is a weird and wonderful take. It plays with the idea of Kuroko's misdirection and Kagami's instincts on a metaphysical level, almost like a soulbond AU. The prose gets a bit abstract at times, but the connection it builds between them feels earned and strangely fitting for the 'Shadow and Light' theme. I remember reading it years ago and it still pops into my head sometimes.