I'd say either Madison or Jefferson (I voted Madison). Madison because he wrote most of the Constitution (and the Bill of Rights if I am correct which greatly influenced states to support the Constitution). And, even if you think he handled the War of 1812 in the wrong way, he did save the United States from ultimate devastation by Britain, the world's superpower.
Jefferson, because of his influence on the nation's growth. Seriously, the US without the Louisiana Purchase? That kind of set the ground for Manifest Destiny. Without the Purchase, there's a chance Napoleon would not have had what he needed, and fall earlier.
Both because of the Kentucky and Virginia Resolutions. They contributed to the whole idea of states' rights. Such, shot by slavery, started the Civil War. That destroyed slavery and states' rights, giving us our America. You could say they destroyed what they supported.