Humans, the fear/madness/emotions that humans give off, other Eldritch Abominations, the works.
However, that doesn't mean they won't each some human food to keep appearances when they have a physical form.
This needs a big dose of sciencing.
Do Eldritch horrors need the human flesh, or only the terror of that happening to the victim?
Test: eat human flesh samples, unconscious human and live human to compare.
Potential: No Tokyo Ghoul horror stories.
Does eating the emotions remove them?
Test: Sit sad test subject next to baby. Instruct to feed.
Potential: Baby is a sadness vacuum, creating happiness everywhere.
Do different emotions have different tastes/effects?
Test: Simulate a range of motions and instruct to feed.
Potential: Baby is the best psychologist, can literally taste-test a person's mental state.
What happens to human food eaten when in human form? Is it digested? Are nutrient going anywhere?
Test: Feed. Wait. Clean up.
Potential: Big savings on baby food.