I use Formula One fish flakes by Ocean Nutrition. It is one of the highest quality flake foods available. Check out how most of the first ingredients listed are nutritious meats instead of cheap fillers.
Ingredients
Salmon fillets, Euphasia pacifica plankton, squid, wheat germ, tuna eggs, Euphasia superba plankton, wheat flour, sea clams, kelp, adult brine shrimp, brine shrimp nauplii, soya-lecithin, MPAXTM (Marine Protein Amino eXtract: fish meals, select amino acids (DL-Arginine, L-Lysine, DL-Methionine, Betaïne, TL-Tryptophan)), vitamins (ascorbic acid (Vitamin C), biotin (Vitamin H), Vitamin B12, riboflavin (vitamin B2), thiamine mononitrate (vitamin B1), tocopherol acetate (vitamin E), panthotenic acid (Vitamin B5), menadione (Vitamin K3), folacin (Vitamin B9), cholicalciferol (Vitamin D3), niacin (Vitamin B3), retinol (Vitamin A1), pyridoxine (Vitamin B6)), minerals (potassium iodide, iron oxide, manganese sulfate, magnesium oxide, zinc sulfate), ethoxyquin, carotenoid pigments.
I also use Cyclop Eeze in the freeze dried form. The wafers are another good choice. And of course the frozen like Mitch mentioned but that's just a bit too much hassle for me.
CYCLOP-EEZE®: Product Info Make sure you get the Cyclop Eeze and not the knock off, "Cyclops", by San Francisco Bay. It is NOT the same thing at all! Here is a good place to get it online.
Liquid Life Cyclop-eeze Whole Freeze-Dried 30 Gram
I also use some good old fashioned Spirulina and Chlorella in the mix.