The ideal outline form of a rest seat is triangular because it provides maximum stability and support by distributing occlusal forces along the long axis of the abutment tooth. This shape ensures that forces are transferred efficiently, minimizing stress and preventing tooth damage. Additionally, a triangular rest seat helps in maintaining the path of insertion and removal of the prosthesis. The triangular form also engages the marginal ridges and central fossa, enhancing retention and stability.

Reference: Prosthodontics, Shigli, Edition 5.