Antonyms for the word "most dissolvable" include "least soluble," "insoluble," and "undissolvable." When we talk about something that is "most dissolvable," we often have in mind substances that can be dissolved easily and completely in water or other solvents. However, there are many materials that are much harder to dissolve or may not dissolve at all. For example, rocks, metals, and plastics are typically considered insoluble or undissolvable. While most dissolvable substances tend to be liquids, such as salt or sugar, there are exceptions. In general, it is important to consider both the solubility and insolubility of materials in order to understand their properties and behavior in various contexts.