George Washington was unanimously elected by both the federalist and anti-federalist parties not once but twice. Washington is generally held by historians to have run unopposed in 1792. Indeed, the incumbent president enjoyed bipartisan support and received the first vote of every elector. He was a war hero and shunned royalty in favor of merit. Not only that, but he basically started many of the customs that presidents still follow to this day. His warning against foreign entanglements was heeded by future presidents, and played a huge role in defining American foriegn policy until 1898. By declining a third term, he also set up the precedent of a two-term limit. For these reasons and more, I suggest that there has never, and never will be again, a president so well-liked as the father of our nation, George Washington.
Truman had the highest single approval rating of all-time (87%) but Kennedy was the most stable and had the least amount of variance between highest/lowest rating (27% difference).