I just did a google search for this and the first site says the three healthiest fish to eat are …
-Rainbow trout (farmed)
-Striped bass (farmed)
This is all based on the on how many nutritional perks you get from them (like amount of Omega 3, selenium (a mineral that bolsters cancer-fighting antioxidants) and various vitamins) but also takes into account what toxins the fish can have in them because of their habitat (which is why some are better farmed and whilst others are better wild).
So like Jessica said salmon wins! It actually wins by a long way having a huge amount of healthy stuff in it. Researchers even think it could help you maintain brain health as you get older and prevent the onset of dementia!