Any kind of meat. It's funny to watch--I'll have an old sandwich or some scraps that include other things, and I'll stand there and tear off pieces. The bread, greens, fruit, etc., they leisurely peck at and eat as I tear and toss them down. But as soon as I tear off a piece of meat--turkey, beef, fish, or (gasp!) chicken--they can smell/see/feel it coming and they rush in on the attack before it even hits the ground! Vicious carnivores!
We ate some corn on the cob yesterday and threw in the leftover cobs. They went crazy and fighting each other for them. The next time I went outside they came running for more.
From my experience, they love just about ANYthing edible and otherwise. I don't think anybody mentioned eggs, did they? Mine liked them scrambled or hard boiled plus all the other things mentioned here. Apples and apple sauce were also on the list! Why, they'd even eat small snakes and mice but that was only when they found one...I never suppled them with that particular "treat"! LOL