When it comes to identifying synonyms for the term "best-kept," the following terms may come to mind: well-guarded, closely guarded, closely concealed, highly confidential, closely protected, and discreetly preserved. Each of these terms indicates the idea of something that is carefully hidden or kept safe and secure from prying eyes or unwanted attention. Such terms can apply to both physical possessions as well as information that is deemed sensitive or confidential. Whether in reference to a hidden treasure or a closely guarded secret, using these synonyms for "best-kept" helps to convey the idea of something that is highly valuable and well-protected.