For cognitive functions to improve, is fish really good? And if so, what type of fish is better than the others?
Any kind of fish is fine, it's higher in minerals. Remember the brain uses a lot of metallic minerals to relay light and conduct electricity That's how the brain and nervous function.
The mineral concentrations in fish are far greater than any other food, especially in oysters and clams and shellfish, so those are your preferable.
Eggs are very important to cognitive function, not because of the mineral concentration, but because the available very tiny, easily digestible proteins from the egg white.