Answer
The right wingtip, independent of direction.
Work Step by Step
Imagine a passenger in the plane, and consider positive charges in the wing. They are moving forward with velocity $\vec{v}$, inside a magnetic field $\vec{B}$ that points upward. By the right hand rule, they feel a magnetic force $q\vec{v}\times \vec{B}$ to the right. The rightward movement and accumulation of positive charges raises the electric potential of the right wingtip.
The argument did not depend on the compass direction along which the plane was flying.
