This is the oldest and most traditional processing method passed down from our ancestors. Sun-drying means letting the coffee cherries dry directly under the sun for 2-4 weeks. During this process, the cherries must be continuously turned to ensure even drying. This method allows the beans to slowly "absorb" the flavor from the pulp, resulting in the most robust taste.
If you prefer complex, rich, sweet, fruity, or even slightly wine-like flavors, sun-dried is the way to go.