One possible synonym for the phrase "most septemviral" could be "most significant." This term suggests that whatever is being described is highly important, pivotal, or central to the matter at hand. Another possible synonym could be "most prominent," which highlights the idea that the thing being described is particularly noticeable, visible, or noteworthy. Other options might include "most crucial," "most pivotal," or "most salient," all of which suggest that the thing being described is critical to understanding the broader context or issue in question. Ultimately, the choice of synonym will depend on the specific context and desired connotations.