The exercise considered best for core strength is plank variations. Planks engage multiple muscle groups including the rectus abdominis, obliques, and transverse abdominis, providing a comprehensive workout for the core without straining the back. They enhance stability, improve posture, and aid in functional fitness. Other effective core-strengthening exercises include dead bugs and Russian twists, but planks remain a foundational choice due to their versatility and ability to target deep core muscles effectively.