Admiral

Admiral: 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 Admiral served under the World Government.

List of Admirals
Only 3 Admirals are seen in One Piece serving in the Marines under Sengoku

Referance
The usage of flag ranks in One Piece is generally based on real-world conventions. Also, the lack of categorical references to full Admirals in the Marines cannot be countermanded by appeal to real-world standards: as the evolution of flag ranks in the United States Navy shows, scenarios are possible in which only part of the usual hierarchy of Admirals' ranks is used.