Probably Carter or Bush...ironic considering how they left office. But between Bill mostly propping up Hillary's career and health issues and Obama's first years marked by a vengeful successor...well those kinds of things complicate things a bit.
In addition to all of this, Poppy Bush almost got assassinated after leaving office, so probably Carter by default. He probably is the most happy though, too, as he seems to genuinely enjoy what he's done since leaving office (sunday school, Habitat for Humanity, etc.).
