First off, I don't seriously doubt that light is quantized. I'm just curious to know the proper historical development of this non-trivial fact of reality (well, now it's trivial, to some people at least).
I've always believed that the cornerstone pieces of evidence for the quantization of light were Planck's blackbody radiation curve, the photoelectric effect, and the Compton effect. But not too long ago I learned that both the photoelectric effect and the Compton effect have semi-classical explanations - i.e. functional explanations that assume light is not quantized.   However, Planck's blackbody radiation curve still seems to be perfect evidence for the failure of the classical theory of radiation.
My question is the following: exactly how did we learn that light is quantized, and rule out all [semi-]classical explanations? Please don't give me the standard "Planck showed that the quantization hypothesis led to the correction of the UV catastrophe... the photoelectric effects shows us that the energy of light is proportional to its frequency not its intensity... Compton scattering shows that light follows the same relativistic kinematics as a point-particle...". I've read that already and given links that mention why the latter two don't necessarily provide "indisputable evidence". Certainly there are hundreds of other historically important papers that are going under the rug here.