Pike is by far the fastest fish. However just like the Cheetah on the savannah it cannot maintain this speed for long. It is after all just like the Cheetah a predator. The pike is perfectly designed for massive speed in short bursts.
In the UK I reckon pike too. They can reach up to about 20mph when attacking prey. For constant speed i reckon Salmon. But obviously no where near as fast as marine fish such as Sailfish that reach about 68mph
I agree tench master, although lance those bonefish we're on about are living in 1 foot of water, believe me 5lb carp in 2 foot of water all the way across absolutely rip it off. Last year this happened several time, I hooked a carp down the inside, instantly Im just hanging onto the pole as there is 40 meters of elastic pulling out and still pulling although its bottomed out!