John Mayer has used several different instrument cables throughout his career, but he's known to be a fan of Mogami cables. Thse cables have conductive polymer sub-shield for great functionality. Mayer has also used cables made by Evidence Audio, designed to provide a clear and transparent sound with minimal noise and interference.
Ultimately, the best instrument cable depends on your preferences and playing style. Still, high-quality cables like Mogami and Evidence Audio can provide a clear and balanced sound.