The Supermarine C.5 the Fleet Air Arm might have had

Which unbuilt British naval aircraft is the most compelling design?

The original Supermarine C.5, because a 1940-era Fleet Air Arm fighter of that type might have changed Mediterranean operations.

The original Supermarine C.5 is the preferred British naval aircraft that never reached service. The design was expected to become available around 1940, when the Fleet Air Arm badly needed a modern fighter. Its timing would place it aboard British carriers during the difficult early Mediterranean campaigns. A capable C.5 could have changed fleet air defense and the balance of carrier operations in that theater.