Matter behaving like a wave rather than like a particle is best illustrated by which phenomenon?

The photoelectric effect is about electrons being ejected from metals when light is shined on metals. The electrons do not behave like waves in the photoelectric effect. Black body radiation is all about the radiation emitted by warm bodies and not about those bodies behaving like waves. The emission spectra of atoms is all about what light is given off by atoms when electrons in those atoms jump down to lower energy levels from higher levels. That also has nothing to do with matter behaving as a wave. Interference is classically defined as the generation of a new wave with an amplitude modulated according to the waves that interfere to form that new wave. Note its emphasis on the wave part.