Most sustentative is a term that refers to something that is highly nutritious or provides great support. When looking for synonyms for this phrase, some suggestions may include: nourishing, wholesome, substantial, enriching, sustaining, fortifying, and invigorating. Another consideration could be using alternative phrases that convey the same idea, such as "most beneficial" or "most essential". Regardless of which synonym is used, the overall meaning of the phrase remains unaltered, and the term can still convey a sense of providing significant value or support to something or someone.