Looks like it would be Romney or Guliani.
Americans don't like to elect senators and congressmen to the Presidency. Surprisingly insightful, the American electorate likes to elect people to the supreme executive position who actually have management experience. There has only been one congressmen elected President directly out of the House (Garfield) and there have only been 2 senators elected directly out of the senate: JFK and Harding, all three of whom died in office. Hmm... maybe we should be rooting for Hilary....