Gotta be GW. The man was so universally loved that he never had to campaign and faced no real opposition in his two elections. And as the first President, he was pretty much free to do what he wanted, unconstrained by precedent or public opinion (he conveniently invented the custom of "executive privilege" when it suited him). Unlike almost everyone else to hold the office, he stepped down as beloved as he had been when he arrived.
Only downside: no White House to live in.