actually the best thing for an anemone is nto food its the light that the photosyntheic bacteria need. The organisms in teh anemone produce the food that the anemone needs to survive.
In addition to what it gets naturaly from photosynthesis i would say silversides.
halides and water quality are the most important thing, but i'm assuming you mean to feed it directly.
small meaty foods work well. anemones can't eat very big pieces though. the food has to be chopped so it is smaller than the mouth of the anemone. you can't assume that if it eats a big chunk of meat that it will keep it down. And No Goldfish!