A good breakfast, in my experience and readings, is mostly fiber, with some fat, and a little naturally-occurring sugar. Peanut butter toast and a banana does that perfectly. Yogurt with honey and a banana. Bread, cheese, and an apple. A boiled egg and butter toast. Cold leftover pizza :)

The fiber is most important, to provide some grist for your digestive system. The fat quenches some hunger and allows you to put off your next meal/snack with less discomfort. A little sugar provides some early energy.