The phrase "most rainbowlike" can be substituted with a few different synonyms depending on the context in which it is being used. For example, "vibrant" or "colorful" could be used to describe something that is most rainbowlike in terms of its bright hues and tones. "Irregular" or "prismatic" could be used to suggest something that has the same kind of fanciful, sparkling quality as a rainbow. "Multicolored" or "polychromatic" are other synonyms that could be used to describe something that encompasses the many colors found in a rainbow. Ultimately, the most appropriate synonym for "most rainbowlike" will depend on the specific meaning and context of the sentence where it appears.