The answer really depends on how your morning routine goes!
you should wait to brush your teeth for at least 60 minutes after you have eaten. If you are able to eat breakfast and still have time to wait before you leave the house to brush your teeth then brushing after is fine! However, most of us are in a rush in the morning so it would be best to brush your teeth before breakfast and here is why!!
Brushing your teeth when you wake up gets rid of the bacteria that was forming through the night (yes it is still there even though you brushed the night before!).
It is better for your enamel not to brush directly after eating anything acidic. Some breakfast items that are acidic include coffee and grapefruits. If you are looking for a way to reduce acidic foods in the morning, yogurt is a great option! Even if what you are eating is not acidic it is still best to wait before brushing your teeth! Brushing your teeth right after you eat something acidic will rub the acid into the porous enamel of the teeth and this will weaken it, which can cause permanent damage over time. By waiting to brush it gives your mouth time to neutralize and wash a way the acids.
This also is a good rule of thumb for brushing your teeth after dinner. Be sure to wait at least an hour before you brush your teeth!