If we are at the equator on the surface of an atmosphere-free, non-rotating, perfectly spherical planet (no mountains or trees, etc) then the optimal angle should be zero (launch towards the horizon, tangent to the planet's surface).
This assumes the initial velocity is the orbital speed at the surface of the planet, which is approximately the escape velocity divided by the square root of two. The range is then infinite, assuming by "range" we mean the difference between the place of launch and the place where the projectile hits (it never hits the ground, in this idealized situation). This is based purely on classical mechanics.
If by "range" we mean getting as far as possible from the planet, then the initial velocity needs to be escape velocity so that the projectile escapes from the planet entirely. If the planet is not rotating then I believe the direction of launch does not matter in this case.