Temperature
The highest possible temperature is the Planck Temperature. When things get hot, they glow. First red hot, which has a long wavelength, and eventually blue, which has a short wavelength. The hotter they are, the shorter the wavelength of light is that they glow at. At the planck temperature, this wavelength is as short as possible. It should be noted that more energy could be added to the system from here, however we don't know what would happen. The planck temperature is extremely high, at 141,678,400,000,000,000,000,000,000,000,000 K or °C, or 255,021,120,000,000,000,000,000,000,000,000 °F.