The antonyms for the word "most paper-thin" would be thicker, denser, substantial, sturdy, or robust. These words indicate a level of strength and durability that is the opposite of something that is paper-thin. Such materials can be thicker, tougher, and more weighty, providing a substantial structure that can resist damage more effectively. Though paper-thin materials may have some advantages in certain situations, the opposite qualities of thickness and strength are more desirable in many other contexts, such as construction, machinery, and electronics. The antonyms for "most paper-thin" show that there is a wide range of materials with different properties, and each can be used appropriately for different purposes.