Growing up, it was considered common knowledge that John Kennedy was one of our greatest 20th Century presidents, cut short in his prime just before he was able to end segregation and poverty and bring freedom to the world. This is because I was surrounded by hippies who had their heads up their asses--JFK was an overrated fancy-pants who dragged his feet on a civil rights movement that was taking place with or without him (his successor LBJ actually got on board that train), his bungling of Cuba in the Bay of Pigs directly caused the missile crisis the following year, and he got things rolling in Vietnam in a way that destroyed the following two presidencies. I shudder to think what another term might have done with that clown at the helm.
A writer for Foreign Policy Magazine seems to agree, and takes it a step further to say JFK was "the worst" of our 20th Century presidents. This is a harder sell--after all, Woodrow Wilson got us into another unnecessary war, and was our most segregationist president of that century. But my vote still goes to FDR for the following reasons:
1) Picking a war with Japan that could have been avoided. We cut off their oil supplies as a protest against their war in China--keep in mind we didn't have any issues with European powers invading that country (hell, only forty years earlier we were among those invaders). Continuing to work diplomatically with Japan may have emboldened moderates like Prime Minister Konoye, and worked out respective spheres of influence in Asia. Playing hardball at a time when the U.S. was trying to support the British against Hitler was stupid and provocative--it put Tojo in power and forced Japan to choose between economic and military ruin or war with us.
2) Happily handing over to Stalin everything he wanted, including half of Europe, when Stalin was heavily dependent on Allied aid. Since the whole war started over Polish sovereignty, giving all of Poland, plus everything else behind the Iron Curtain, to a dictator who could teach lessons in cruelty to Hitler, made WWII a sort of hollow victory and was the proximate cause of the Cold War. The argument that he "had no choice" sort of rings hollow--Stalin had no choice but to fight Hitler, and the Allies had the upper hand in the alliance.
3) Refusing any help for Jewish refugees from Hitler. FDR did not allow Allied planes to target rail links to the death camps, despite the fact that these targets were in their flight paths on their normal bombing runs, and he refused to allow open immigration of refugees into U.S. territory (or use any diplomatic pressure on the British to take in the refugees). FDR apologists argue that he had to do what he could to win the war, but none of these possible moves would have hampered the war effort in any meaningful way (unless you argue that Nazi bullets used on innocent people meant the bullets couldn't be used against Allied troops). Sadly the Italians while Mussolini was still in power did a better job protecting Jews than FDR did. The only plausible argument for FDR on this score is that he couldn't believe the information he was getting about the fate of the Jews in Europe.
4) Intentionally targetting civilians with firebombing. While in Europe the Americans' bombing efforts were limited to military targets (airfields, rail links, bridges, factories), we used firestorms against the Japanese on their home islands. The firebombings of Tokyo killed far more people than the atomic bombings would, and these weren't "collateral damage"--it was our intention to kill and terrorize the local population so that they would go to the polls and vote against the war. Oh wait, this was a dictatorship and they wouldn't be voting at all? Whoops! I guess we're just killing innocent civilians for the lolz! This actually didn't help us win the war--any more than Hitler's bombings of London and Coventry made the British want to end the war. It just emboldens the population, and kills a lot of noncombatants. We execute war criminals for that sort of thing. Of course, only the winner gets to do that. The only counterargument for the firebombings is that FDR thought at the time that it would cow the Japanese into surrender, and that it would take something more awe-inducing (like the atom bombs) to actually do this. But the firebombings themselves turned out to be pointless mayhem and unjustifiable with the light of history--and it is with that light that we judge past presidents.
5) Civil liberties. Who was our only 20th Century president to round up tens of thousands of American citizens and their families, seize their possessions and homes, and put them in holding camps far into the interior of the country, with no trials, etc.--only because of their race? Yep, that civil libertarian FDR! Any Japanese-American who doesn't hate that guy really needs to read more. And wartime censorship was notorious in a way that would make Michael Moore blush if he weren't a stupid moron.
6) The Counterargument. I won't go into the economic and domestic policy arguments, since there's still a lot of argument as to the effects of these policies, as well as the fact that these policies ran the gamut. Some revisionists argue that his policies prolonged the depression (and caused the "double dip" in 1937, though contractionary monetary policy also takes some blame), others argue it had no real effect (as unemployment was still around 20% at our entry into WWII), and of course those on the left believe he lifted us out of it or at least helped prevent it from getting worse. None of that can be proved--not without an alternate reality where those policies never happened--and it will always depend on your economic standpoint. I'll credit him though with the fact that he took office at a horrible economic time, and tried pretty much everything--and a lot of it did have a good effect (rural electrification, creation of deposit insurance). He was no great civil rights leader for the black population--since his coalition was dependent on segregationist votes--but his presidency wasn't a step back either. So he gets a wash on domestic policy.
FDR apologists will also say that he recognized the threat from Hitler, and did everything possible to get involved in the war despite isolationist sentiment at home. This is true--a different-minded president may have stayed out of it (as I argue Wilson should have in the first war--though in the first war, blame for who was responsible was a lot more muddied than in WWII) until forced into the conflict by Britain's collapse. But once that decision was made, his bunglings made things more destructive and longer than they needed to be, with a worse outcome than we otherwise might have gotten. We did not need to fight Japan--and we did not need to give Russia control of Europe up to the Elbe (at least, without anything in return). We did not need to insist on "unconditional surrender"--a policy that prevented anti-Nazi plotters in the German high command from getting support within the Army to overthrow Hitler and sue for a negotiated peace. We also might have prevented some of the millions of death camp fatalities, and certainly did not need to target enemy civilians in our own attacks. And it's not hard to imagine that another U.S. president could have successfully fought Hitler without making all those mistakes.
With the "Monday morning quarterbacking" of history, it leaves little doubt that FDR did more damage than any other 20th Century president. So why do we have a memorial to this monster?