Something good to keep you going and fuel you through your hard work.
Preferably also something you're used to (and enjoy) eating, so that you know it will "sit well" and not leave your belly rumbling or gurgling in the middle of an exam!
And make sure you have some lunch (and dinner) as well – gotta look after yourself so you can do your best.
My usual breakfast is porridge with berries, banana and peanut butter plus any extra toppings I can find in the cupboards.
Some other quick ideas:
Bagel (or a few slices of toast) with peanut butter and banana, peanut butter and jam, butter and jam, avocado and eggs, cream cheese and smoked salmon, butter and scrambled eggs…
Yogurt with granola/muesli and fruit
And have something to drink with it as well – water, fruit juice, tea, etc.