Human flesh, they have also been known to eat human bones as well strangely enough According to the Draconomicon (Wizards of the Coast, 2003): "Dragons are carnivores and top predators, though in practice they are omnivorous and eat almost anything if necessary. A Dragon can literally eat rock or dirt and survive. Some Dragons, particularly the metallic ones, subsist primarily on inorganic fare. Such dining habits, however, are cultural in origin."
Also, Dragons are quite partial to the flesh of virgin human females. It is unknown why they prefer virgins, though most speculate that it is simply because they taste better.