Admiral

Admiral (大将, taishō) is a title of admiral signified senior command rank in the Marines. Admirals, also known generally as flag officers, were typically responsible for a high level military command at battle group or fleet level, and/or for administrative and political duties relating to Marine operations.

Pirate Admiral
The only known pirate Admiral is Don Krieg who commanded 50 flag ships with two of his commanders Pearl and Gin

Marine Admiral
The Marine Admirals serve under the World Government.

List of Admirals
Only 3 Admirals are seen in One Piece serving in the Marines under Sengoku.
 * Aokiji
 * Akainu
 * Kizaru