--Great--
1. Franklin Delano Roosevelt - FDR did many bad things (court-packing, Japanese internment), and had others that were screw-ups (misjudging Stalin, some of the herky-jerkiness of the New Deal) but ultimately he kept the US democratic in a pretty dark time. He also did everything he could to support the Allies before Pearl Harbor, which was tough against an isolationist Congress, but nevertheless might have ensured that there was a war left to win after 41. (In a full ranking, Roosevelt would be the only one hanging out with Washington, Lincoln, and Jefferson in the Greats.)
--Excellent--
2. John F. Kennedy - No I am not a baby boomer. An anti-communist without going over the edge, Kennedy did a good job in his short time in office - dealt with the Cuban Missile Crisis effectively, and lightly helped South Vietnam. He set the stage for the Civil Rights Acts to be passed later, and he also kickstarted serious space exploration, which is probably what he'll be remembered for 500 years from now, the Queen Isabella to Armstrong's Columbus. Okay yes I am a NASA fanboy which is why he's so high up here.
3. Theodore Roosevelt - The man. For such a fire-breathing hawk, it's lucky that the world situation never let him get into any bad idea wars, but he was actually a pretty decent peacemaker. Taking a shot at the robber barons was also needed.
4. William Howard Taft - Policy-wise, Taft is Roosevelt, just less manic and charismatic. Also busted trusts (unusual for the Republican party, even at the time!), started protecting the environment, and so on.
5. Dwight D Eisenhower - A surprisingly solid president. Like Kennedy, he also pushed civil rights forward, and later on space flight. Appointed Earl Warren to the court (even if Warren turned out way more liberal than expected) and generally was competent. Main flaw: Focused on nuclear weapons as the main US Defense Arsenal because "more bang for the buck." Um, yes, if we're willing to fire them, which we won't be once the Russians can nuke us back?
6. Harry S Truman - Not as much on the Truman hype train as some people. He gets points for courage (pushing a civil rights bill in 48, an election year!) and for firing McArthur, as well as not turning the Korean War into WW3 against China, but.... I don't know, it just seems like Truman should have been able to set up the Cold War world better than it turned out. The Marshall Plan was good times, as some of the other aid to anti-communists, though.
7. Ronald Reagan - As a liberal, I'm not really a fan of many of Reagan's domestic policies. I guess some of the 60s/70s social program splurge did need to be trimmed, but meh. He was an effective leader abroad, I'll grant, and he had the charisma to pull off the impressive feat of insulting the USSR and pointing out it was a tyrannical mess (needed to be done) while also saying "let's make a deal." Much like Clinton, Reagan has to deal with a Democratic Congress his time in office, which was certainly a good thing - the sillier parts of his programs were restrained, and Reagan was a far better president as a result.
8. Woodrow Wilson - I read an interesting theory in a book which is that rather than a stroke at Versailles, Wilson actually got the brain-eating flu instead. Which would explain why he rolled to the Europeans and was unable to really fight effectively for passage of the League of Nations at home. A tragic coincidence in history that the most forceful and charismatic proponent of ending WWI right was, in some sense, worse than killed. Anyway Wilson was totally right about things like self-determination and being suspicious of the "mandates" (read: new colonies) that England and France wanted, but he kind of failed to stop them (thanks to the flu). On the bright side, Wilson set up the Federal Reserve, ending some of the monetary madness that was in the US in the late 1800s. On Civil Rights... Wilson is very mixed. He did nominate Brandeis, a Jew, to the Supreme Court about thirty years "early" and force it through... and he was certainly sympathetic to oppressed European minorities controlled by distant empires (the Poles, the Serbs, even the Arabs, etc.). On the other hand he was a total racist against those not white enough, shrugging off Ho Chi Minh at Versailles (yeah this will never come back to haunt us) and re-segregating the federal government and Washington DC. And generally kicking blacks to the curb, publicly praising "Birth of Nation," yeah. Like Nixon, hard to rate, though on the balance comes out okay.
--Competent--
9. George H. W. Bush - Generally an adult even if he pandered on some issues. Realized that the US needed to transition well to a unipolar world, and withdrew a lot of US aid from various dictatorships that we'd been propping up strictly because they weren't commies. A fateful decision with mostly good consequences (though it can be argued that abandoning Afghanistan might have been the one place it backfired). Also was mature enough to tear up his campaign promise and raise taxes back when congressmen actually took balancing the budget somewhat seriously after the expenses of the Reagan years.
10. Bill Clinton - Clinton's hard to rate. He screwed up his first two years and then faced an extremely hostile Congress the other six. I'd argue that divided government is generally a good thing, but it's hard to say how much of Clinton's proposals were Clinton and how much were compromises in order to get anything done. He was a masterful politician, I'll give him that, and he stopped the Republicans in Congress from doing anything TOO stupid (unlike, say, 2002-2006). Also he helped liberalize the policies of the executive branch in a lot of subtle ways.
11. Lyndon B. Johnson - An extreme mixed bag. Made some of the triumphal liberal legislative achievements of his time, notably including Civil Rights Acts with teeth, a feat that was only possible thanks to Johnson's mastery of Congress. Some of Johnson's social programs were also definitely needed, stuff like the Food Stamp Act. On the other hand, the fact he was able to get whatever he wanted done allowed him to slip some bad programs in there, and then there was intensifying the US in Vietnam. Ugh.
12. Calvin Coolidge - Didn't really do anything, but did nothing efficiently and non-corruptly? I guess you could blame him for failing to ward off the Depression and not doing anything about Prohibition. Which I do, but only lightly. Also he helped pass the Kellogg-Briand Pact which OUTLAWED WAR. Yeah that worked well.
--Disappointment--
13. Gerald Ford - Nice guy. Pardoning Nixon was even the right thing to do. But generally flailed around. Efforts to fight inflation were laughably stupid (check out Wikipedia for Ford's "WIN - Whip Inflation Now" buttons. Yes that will cure our problems while the Fed Chief is pouring gasoline on the fire with too-low interest rates.)
14. Jimmy Carter - See above, actually. Nice guy who didn't get much done, except toss in some bad taste in friends / cabinet appointments that forced Carter to fire half his cabinet halfway through the term.
15. William McKinley - Not really a 19th century guy. Main achievement was temporarily resisting the Spanish-American War, then fighting it anyway... hmm. He did do the right thing in Cuba, but kind of messed up the Philippines. Hard to say.
--Fail--
16. Richard Nixon - Canonical example of a president hard to rate on a single scale. The American Macbeth may have actually been a somewhat decent president (deepen the separation between China/USSR, start the EPA), but he had his flaws too (ineffective price controls, and set up the inflation that Ford & Carter would have to deal with). Also there is the whole megalomaniacal "the President is the President and thus can't break the law and gets to do whatever he likes to his enemies because he's SAVING THE COUNTRY." Maybe the foreign enemies, but not the domestic ones.
17. Herbert Hoover - The only two Quakers to be President? Nixon and Hoover. Oops. Dave Barry pointed out in one of his books that Smoot-Hawley is an inherently funny word, even if the tarrif was a bad idea.
18. Warren G. Harding - His SHINY PANTS are IRRESISTIBLE to women. No I'm not making this up. Also he sucked as president and dropped the ball on the League of Nations and failed to notice extreme corruption in his cabinet go die early kthx.