Anything starchy; bread, potatoes, pasta, or anything with sweeteners.
Pure glucose would be the most dense. A carbohydrate is a sugar, composed of carbon hydrogen and oxygen. We eat two forms of carbohydrates, simple (monosaccharides) and complex (polysaccharides). The quicker the carbohydrate is converted into glucose, the seemingly "worse" it is for our bodies. This is measured by the glycemic index. Foods high on carbohydrates are pure simple sugars (sucrose or table sugar, fructose which is found in fruits, lactose from dairy products,etc). HFCS or high fructose corn syrup is one of the highest on the GI scale. Complex carbohydrates are found in starches (refined white flour is a good example).
Here is a listing of foods by caloric content. It lists homemade dark fruitcake at the top of the heap.
Sugar has the highest level of carbs because it is all carbohydrate.