One popular kokichi x shuichi fanfic is 'A Bond Beyond Doubt'. It beautifully portrays their relationship development with great character insights.
There's a fanfic named 'The Unlikely Alliance' which is really good. It delves deep into the complex dynamics between kokichi and shuichi, showing how they overcome various obstacles together. The author does an amazing job of building the tension and chemistry between them.
Well, 'The Shared Journey' is also a great kokichi x shuichi fanfic. It starts with their initial encounters and then gradually unfolds their relationship, exploring their individual personalities and how they complement each other in different situations. It's full of emotional moments and interesting plot twists.