The best lingual rest for a removable partial denture is placed on a prepared rest seat on a cast restoration because it provides a precise, stable, and properly contoured surface that can effectively distribute occlusal forces. Preparation ensures the rest seat has adequate thickness, correct angulation, and positive seat morphology, which preserves tooth structure and prevents trauma. Cast restorations offer better control over rest design and increase the longevity of both the tooth and denture. This method also enhances retention, stability, and patient comfort by reducing movement and stress on abutment teeth.

Reference: Contemporary Fixed Prosthodontics, Rosenstiel, 5th Edition.