The word "most voyeuristic" is commonly used to describe someone or something that takes pleasure in secretly watching other people without their knowledge or consent. Some synonyms for this word could include "peeping tom," "snooper," or "spy." Another possible synonym could be "creep," which implies a sense of discomfort or unease with the behavior being described. Other related words might include "intrusive," "nosy," or "prying." Although these terms may have slightly different connotations or shades of meaning, they all convey a sense of fascination with observing others from a distance or in secret.