The phrase "most deep down" can be replaced by several synonyms that convey a similar meaning. One common alternative is "innermost," which refers to the most personal, intimate, or essential aspect of oneself. "Inmost" is another similar word, which denotes the deepest or most secret part of something. "Core" and "heart" are also appropriate synonyms that describe the central or fundamental element of a person, idea, or situation. Other possibilities include "root," "foundational," "ultimate," "profound," "bottommost," or "fathomless." Choosing the right synonym depends on the context and tone of the sentence, but any of these words can add depth and clarity to the message.