Eggs and some kind of protein (sausage or bacon, traditionally) is my personal recommendation, you can do them in breakfast burritos or sandwiches as well to get some carbs (which are good for energy, just be-warn large amounts of carbs tends to lead to weight gain), and if you take the burrito or sandwich route you can easily add peppers, mushrooms, onions, cheese, or whatever else you want, for variety or just because you like them.