A variety is best. Buy 3 or 4 different foods and rotate which ones you use. Mysis shrimp, Emerald Entree and Marine Cuisine are all easy to find and are great foods. Rod's Food is a bit pricier and usually only found at LFS, but Rod's Food already has a mix of stuff and is a great all-in-one food.
My fish also eat bloodworms, fortified brine shrimp, tubifex worms, krill and plankton.