The Spitfire - with all those curves it has to be. Later Griffon powered Spits were less attractive, for me, the optimum beautiful shapes were Mk9 - Mk14 (teardrop canopy as well). Mk22 onwards were ugly in comparison.
Mustang - looks great but angular, but only the later ones (D onwards) with teardrop canopies rather than the P40-type cockpits
Both superb aircraft but obviously not the same role, so it would be comparing apples with oranges if we got into comparisons.