What is the best experiment to prove photons are real?

One needs to be aware that EVERYTHING that we know and accept to "exist" (at least in science) is based on the fact that each element has a set of properties and characteristics. We DEFINE them based on those characteristics. The same way with an atom. We have a set of definitions of what an atom is, and when we test or make a measurement, if that thing we're testing has the same characteristics of what an atom is supposed to have, then that is an atom. The atom is defined based on a set of characteristics (energy states, orbital angular momentum, etc.) that require measurements of those characteristics to verify its existence.

Evidence from the photoelectric effect strongly suggests that photons are 'real' and their energy is indivisible.
There is an instantaneous release of photo-electrons (within 10^-9 s) when the incident radiation has sufficient energy (ie has a frequency above the threshold frequency) no matter how weak the intensity of that radiation is.
With very weak intensity (ie relatively few photons) it would take a detectable time to accumulate the required energy from numerous photons - that time lag apparently doesn't exist.
Again, when a given radiation intensity increases the photo-electrons emerge in greater numbers (due to presence of more photons) .. but the stopping potential (measuring the maximum KE) for these electrons remains the same, indicating that they do not  accumulated energy by interaction with more than one photon.