What has the least energy per photon?

The energy a photon carries is in inverse relation to the frequency of its band of light. At the red end of the spectrum are wavelengths of light with ever-lower frequencies and lower energy values per photon. At the extreme edge of this side of the spectrum are radio waves.

The long-wavelength side of the visible section of the electromagnetic spectrum trails off into red light, infrared and microwaves. According to the Goddard Space Flight Center, the radio part of the spectrum covers wavelengths of between 1 cm and 1 km. The photons at this wavelength carry far less energy than at any other part of the spectrum.