The reason I like Northfin food is because of the ingredients.
Fish food, like human food, should list the ingredients. If they say fish meal, it is not usually good quality. Look for a food that names the species of fish the meal is made from, and use of the word 'whole' is even better as this means it's not just the skin, bones and other undesirable bits. For example, I have recently changed from Atisons Betta Food, and that just lists "fish meal", which is a lower grade ingredient.
Northfin Betta Bits contain whole Antarctic krill meal, herring meal and whole sardine meal.
New Life Spectrum Betta Formula contains whole Atlantic krill meal, whole herring meal and whole squid meal.
Omega One Betta Buffet Pellets contain salmon, whole herring and whole shrimp.
Not all products give the ingredients on-line. Hikari's website just says Betta Bio-Gold contains "premium select fish meal". API's site doesn't say anything about ingredients.