What was Ben Franklin's biggest contribution to the United States?

Almost certainly securing French aid for the Revolutionary War effort. He used his status as an international celebrity and his considerable skill as a diplomat to secure a full military commitment for the fledgling nation from a superpower. His domestic contributions during the drafting of our founding documents and his status as an inventor, polymath, and public servant cannot be overstated, but that one action stands above all others in terms of value to the United States, because without it the colonists would very likely would have lost the Revolutionary War.

His autobiography is a short, excellent read. Isaacson's work is probably the best you're going to do for a modern biography, it is very extensive and enlightening. And if you're studying anything about the American Revolution and you don't read what Wood has to say about it you're just wrong, so pick up The Americanization of Benjamin Franklin.