What is the highest insignia in this list?

The highest insignia among the options provided is Admiral. This rank is part of the United States Navy's officer ranks and is the highest level of admiral rank, which designates a four-star admiral. It is important to recognize that the progression of ranks in the Navy moves upward through various levels of leadership and responsibility. An Admiral commands significant operations and plays crucial roles in strategic decision-making at high levels within the Navy.

In contrast to Admiral, the other ranks listed—Rear Admiral Upper Half, Vice Admiral, and Rear Admiral Lower Half—are all lower in the hierarchy. Rear Admiral Upper Half is a two-star rank, Vice Admiral is a three-star rank, and Rear Admiral Lower Half represents a one-star rank. Because of these distinctions, Admiral is clearly the highest insignia in the list presented.