The paper presents a comprehensive analysis of the spectral energy distributions (SEDs) of blazars, focusing on three complete samples: the Slew survey, the 1-Jy samples of BL Lac objects, and the 2-Jy sample of flat-spectrum radio-loud quasars (FSRQs). The authors collect data from radio to γ-ray frequencies and find that the fraction of objects detected in γ-rays is 17%, 26%, and 40% for the three samples, respectively. They compute average SEDs for each sample and identify a remarkable continuity in the properties of these distributions, including the peak frequencies of the synchrotron and γ-ray components, which correlate with each other and decrease with increasing luminosity. This continuity suggests a unified view of the blazar phenomenon, where a single parameter related to luminosity governs the physical properties and radiation mechanisms in relativistic jets. The paper discusses the implications of this unified scheme and presents a detailed theoretical analysis in a separate paper.The paper presents a comprehensive analysis of the spectral energy distributions (SEDs) of blazars, focusing on three complete samples: the Slew survey, the 1-Jy samples of BL Lac objects, and the 2-Jy sample of flat-spectrum radio-loud quasars (FSRQs). The authors collect data from radio to γ-ray frequencies and find that the fraction of objects detected in γ-rays is 17%, 26%, and 40% for the three samples, respectively. They compute average SEDs for each sample and identify a remarkable continuity in the properties of these distributions, including the peak frequencies of the synchrotron and γ-ray components, which correlate with each other and decrease with increasing luminosity. This continuity suggests a unified view of the blazar phenomenon, where a single parameter related to luminosity governs the physical properties and radiation mechanisms in relativistic jets. The paper discusses the implications of this unified scheme and presents a detailed theoretical analysis in a separate paper.