Freshwater - Pound for Pound it is easily the Jungle Perch.
Saltwater - Don't do alot of Saltwater Fishing, but Wahoo are simply the fastest fish in the Sea... But Id love to fight a 1200lb+ B/Marlin one day. If I could tag and release one of them, Id be the happiest man alive.
Merry X-Mas AusFishers!