Foods that would take the longest to digest would be meats such as bacon, hamburger, beef, lamb etc.
Other foods that would be harder to digest and take longer to digest than some other foods are nuts and whole milk hard cheese.
The food that would take the longest to digest would be hard meats such as steak.
The above foods can take around 4 to 8 hours to digest once you've eaten it and your digestion of food still occurs even while you're sleeping.
Foods such as oatmeal, cereal, bread, donuts, pizza etc may only take a few hours to 4 hours to digest.