The word "most robustuous" is a fairly rare term and may not have many established synonyms. One possible alternative might be "most vigorous," which conveys a similar sense of strength and energy. Other synonyms could include "most hardy," "most sturdy," or "most resilient." Similarly, "most robust" could be used to describe something or someone that is tough and sturdy. More poetic or descriptive synonyms could include "most stalwart," "most mighty," or "most brawny." Ultimately, the choice of synonym will depend on the context and intended meaning of the word, but there are many options available to convey a sense of strength and power.