Waxigloo is correct. The first clear evidence for the quantization of the EM field (as opposed to a quantization of matter-field interaction or a semi-classical model) was the Lamb-shift. The photoelectric effect, compton scattering and blackbody radiation were all evidence that a photon model was useful but they didn't strictly require it. The first clear evidence for photons as particles was the Kimble experiment in 1977 which showed antibunching in a Hanbury-Brown Twiss style measurement. Other similar experiments by Aspect and Clauser should also be cited as they were the first to actually create a beam of light containing a single photon (within some experimental caveats).  This stuff should definitely be in the article.  Here is a reference:  Phys. Rev. Lett. 39, 691–695 (1977) Photon Antibunching in Resonance Fluorescence, H. J. Kimble, M. Dagenais, and L. Mandel.