is essential for life, how we choose to feed our aquarium inhabitants depends on their habits as much as ours! Fresh food is, so long as it is clean, the best. Not the half dead artemia or mysis that has been hanging around in your dealer's fridge for a week or so.
Whatever you do, keep variation top of the list

Live food and Fresh Food

Artemia, mysis, shrimps, mussels, oysters
Lettuce (wash thoroughly) spinach, banana is also recommended by many sources.

Frozen Food

Should be the best, but much is not good abot frozen food, suggesting the chain has been broken somewhere. A survey carried out in Germany detected pathogens (disease- causing bacteria) in a number of frozen foods, including well known brands. Another good reason always to defrost the foods in water and to throw that water away.

Dried and Freeze Dried Food

If kept cool and dry, should be vitamin and trace element rich. Some fishes prefer granules, some flakes. Some fish will not eat these at all, and it doesn't have to be the 'difficult' fish, my pyjama cardinalfish have never been interested in dried food. But there again I do have a Salarias fasciatus (jewelled blenny) which eats just everything I put in.
Dried seaweeds seem to be useful for many herbivores.

Liquid Food

Food Supplements

Add vitamins to defrosted foods and maybe dried foods regularly. Some dried foods have added garlic to get rid of intestinal parasites.