Does anyone know what the most valuable fish in the game is?

I personally try to get and keep just 4 fish

Twin-fin Spotanus
Silky Spotanus
Twin-fin Shark
Silky Shark.

This is because there are no fish the will cross and produce these. It is true that a Shark and a catfish give a shark and a Silky and a Tiger give a silky but it is also true that there is only one way to get a Tiger and that requires a silky and there is only one way to get a Catfish and that requires a shark.

But in terms of money the price of these fish is small however it is possible with careful planning, to breed from just these 4 all 441 different fish in the game and all in just 3 generations.