The correct answer to this open question is the following.
I think the best contribution that had the biggest impact on history was the idea of freedom to be an independent nation and establish its own rules to govern a country. For me, Liberty is the most important contribution because that idea allowed people to fight for independence as was the case of the United States that had to fight in the Revolutionary War to defeat the British troops and get independence from the English Monarchy. Or what about the French Revolution. Or even the Mexican Independence movement when they got freedom from the Spanish crown.