Airfoil/Application Design Integration
Each application requires a specific performance from the airfoil; therefore, an airfoil should be designed for each application. No airfoil can be considered "best" in a general sense, only with respect to the specific application (ref.). The airfoil design and the application design are so strongly connected that the overall design process is iterative, as illustrated below. Accordingly, close communication between the application designer and the airfoil designer is required.

Reference
Maughmer, Mark D.; and Somers, Dan M.: Figures of Merit for Airfoil/Aircraft Design Integration. AIAA Paper 88-4416, Sept. 1988. [DOI]
Related
For details on the airfoil design method, see Subsonic Airfoil Design. For real-world examples, see Applications. To understand the design requirements, see Specifications.