Anything with sugar in it. Sugar will give you an instant hit of energy and may make you feel better short term but the spike in blood sugar will make you feel worse in the long run. Stick with protein and complex carbs. Personally I’d have eggs/bacon for protein and healthy fats with baked beans (the ones with no added sugar) for fibre. It’ll keep you fuller for longer and is so much better for you than sugary cereals.
Other then skipping breakfast, you could always have a quick one. Like a protein bar, yogurt, or even a fruit on the go!
Foods with a lot of fat should be avoided for breakfast, like meat and mayonnaise, also avoid too much refined sugar, go for fruits and honey to get energized
Something to avoid for breakfast is refined grains (white bread, pancakes) and sugar added products like syrup, also artificial juice fruit.
Too much sugar is something we should avoid in breakfast as sugar gives you a great boost but depletes quickly, making you feel even more tired
I prefer not to eat sugar for breakfast so I avoid jam, honey or any kind of suger. Oil is the other thing that I avoid it. Cream and butter didn't have a place in my breakfast.