Gamma rays have the greatest tissue penetration ability because they are electromagnetic waves with no mass or charge, allowing them to pass through most materials, including body tissues, with minimal interaction. In contrast, alpha and beta particles are charged and have mass, causing them to lose energy quickly and be stopped by a few centimeters of tissue or even skin. Ultraviolet rays are non-ionizing and absorbed mostly by the skin's surface, limiting their penetration. Therefore, the high penetration power of gamma rays makes them the most deeply penetrating ionizing radiation.

Reference: Robbins Basic Pathology, 10th Edition, Chapter 3, p. 108