Simply the best

collected   cultured

Live foods are the best food for killifish. Period. They may be cultured or collected from a "clean" pond.

The mainstay of many aquarists

Frozen foods - when of high quality - are very good as well.

Underutilized

Killies (with a very few minor exceptions) are carnivores and any lean fresh meat can be very beneficial as well.

Sometimes works well.

If you have a pair of killies that were raised on live foods they may never eat dry food, but if you raise a batch of fish and start them young they may eat it with gusto. Some species seem to take it more readily than others.