The antonyms for "most unshared" are "mostly shared," "widely shared," "frequently shared," "commonly shared," or "generally shared." These antonyms signify that the subject in question is being shared by many people or is easily accessible to everyone. When something is "most unshared," it implies that it's not being shared at all, or is only being shared by a very small group. For example, "my personal diary is the most unshared item in my possession," implies that it's not being shared with anyone, while "the most widely shared book of all time is the Bible," conveys that the book is known and accessible to millions of people.