It's so tough to pick just one, but I'll go with campaign finance reform. The ideal system would ban private financing and use only public financing for political campaigns. In my opinion, this would trigger a trickle down effect where political/corporate corruption decreases and additional political parties become more relevant and legitimate.
Expand the membership of the House of Representatives. 1 house member for 30,000 citizens was the original equation we should bring that back. The ratio is now north of 700,000 to 1
The house has become body of citizens who only really care about getting enough money to run TV ads so they can get reelected.
IMHO, more congressmen would mean more oversight by their constituents. If the ratio of constituents to Congressmen became 50,000 to 1, chances are we all know our congressman and would be able to have a genuine conversation with him/her. AKA Representative Government.
The two biggest rackets in this country are private sector healthcare and tort lawyers. End both. The med mal tort would be completely abolished and healthcare completely taken over by the government and taken away by private insurers who charge 3X as much for the same crap.