It really depends on what you're looking for in a breakfast. Do you want something that is light and quick to prepare, or something hearty that will "stick to your bones"? If it's the latter, then a warm breakfast consisting of protein/carb-based foodstuffs is the best option. If it's something quick, then anything from fresh fruit to granola all work. I typically look for something nut-based when searching for cold breakfast items, as they can be very beneficial for your overall nutrition.