The answer is...the flat iron! The temperature setting is much higher and people tend to hold a flat iron in one place, so your hair gets more direct heat contact, as opposed to the hair dryer, which is constantly moving around.
Flat irons also tend to have higher temperature settings than blow dryers.
So, if you love your flat iron, use it sparingly to prevent damage and breakage. You can also try using a heat protection spray for your hair and using the lower heat setting on the flat iron!