Phosphorescence is the emission of light without heat. When we talk about the "most phosphorescent" object, we are referring to the material that radiates the brightest light in the dark. The antonyms for "most phosphorescent" are dull, dim, faint, and unilluminated. These words denote the absence of light and are used to describe objects that have no or very little glowing ability. They are the opposite of the bright and brilliant quality of phosphorescence. So, when we say an object is dull or dim, we are describing its lack of shine in the dark, and it is the complete opposite of being the "most phosphorescent.