Steve, some may disagree, but I think it's almost more the additives than the food. Having things on hand like Kent's Garlic Xtreme, Brightwell's new AngeLixir, and Fauna Marin's "True Garlic Extract" has done a lot for me.
It could be coincidence, but Brightwell's AngeLixir has gotten a great feeding response from difficult angels and butterflies for me. I'm thinking maybe it could be these "sponge extracts" they talk about.
Beyond that, I've had good luck with both PE Mysis (assuming the fish are big enough), and smaller foods like Arctipods.
For the REALLY hard to break, I've used live California blackworms. The wiggle is irresistible to most fish, although I prefer not to feed live food!