Personally I think it's best to break it up into categories.
Foreign policy had been here and there since the founding, with annexations and the such, but it really started becoming... evil... with Wilson. He helped to create a new concept behind war, one driven by offense instead of defense. FDR continued it to a lesser extent, and the US involvement in middle eastern politics in the 50s was horrible. JFK ramped it up with the Vietnam War in 1962, LBJ obviously continued+escalated this (from memory no wars since Vietnam have been constitutionally declared by Congress). The seventies weren't much better, the 80s were a good contender for the worst decade of foreign policy that century; Nicaragua stands out, as does the US aid to militant Islamists in the middle east to combat the Soviets, and the whole Iran-Contra scandal. The nineties were a bit of a relief, but only when compared to the decade that preceded it: the Gulf War stands out. The 2000s were a horrible way to enter the new millennium: Bush 2.0 cemented the doctrine of preventive war, distinct from preemptive war (the former being blatantly illegal under international law, the latter of questionable legality). 2996 American (some other nationalities) deaths on September 11th began a response that killed well over a hundred thousand Iraqis killed, as well as 4432 US soldiers in the Iraq War. The US elected a President who talked big on change, but in effect has just escalated the foreign policy doctrine of the previous administration (to a similar extent as Bush's second term). The US support of Israel was over a few decades, so I didn't really include it.
Civil rights have generally gotten much better over the past century, largely due to local scale activism. Suffrage in 1913, substantive civil rights legislation in the 1960s, unionization, etc. Prohibition in the 1920s and 1930s was a big hit against civil rights, as was the war on drugs which was waged beginning fairly early in the century, but two figures stand out in this category: Richard Nixon and Ronald Reagan (1970s/1980s). By far the worst decade progress wise for civil rights was the 2000s. Bush practically dismantled the 4th Amendment to the United States Constitution. Wiretapping, torture, Guantanamo Bay, etc. The nation elected a President to change things, but really he's been more of the same on this issue; his justice department even fought the Supreme Court on torture. Oh, I forgot to mention the Japanese internment camps under FDR. That too :P
Social programs have gotten gradually better also, with FDR being a standout in this field. Social Security, Medicare, etc, with polls putting opposition to spending cuts in these programs at around 65%+. Again, FDR is a standout, although it was ultimately the result of activism a decade prior.
Taxes, depending on how you look at them (personally, I see taxes as entirely irrelevant. The US should spend what makes rational sense, then tax whatever is needed to balance the budget exactly. No surpluses, no deficits), anyway, they've certainly dropped, especially for the rich. Taxes as high as 90% for the top marginal tax rate (in federal income taxes, I exclude state taxes, and other taxes because they vary) have dropped down to sub-50% levels (quite easily).
Wages were increasing for the average worker until the mid 1970s. Since then they have roughly stagnated in real wage terms. In contrast, the gap between the rich and poor has skyrocketed. Corporate profits are up almost exponentially. The portion of the wealth held just by the top 1% has tripled since JUST 1980. In the 1950s, by contrast, an increase in the nation's wealth was distributed more equally throughout each income bracket, rather than the top 1% getting large portions of it.
From a constitutionality basis, the Executive branch gained an INSANE amount of power that it was never meant to have, precisely for the reasons that we see today. Wars not declared by Congress, signing statements, etc.
The US has gotten better culturally. The 1930s began the modern music era in my opinion, with performers like Robert Johnson, and the supremacy of Jazz (one of the great American conceptions of the 20th century). The 1950s were brilliant music wise, and the movie industry really started to get fantastic with films like Citizen Kane in the 1940s, and Hitchcock in the 1950s. The 1960s were arguably the cultural peak of the 20th century, with an arguably unparalleled music scene, and a film scene stuck in the transition between two eras. The 2000s were largely the end of the US cultural boom in my opinion, with music being bland and uninteresting, and great films being much less frequent (although Mr. Nolan is doing a brilliant job).
Intellectually the US has gotten better over the century, no doubt. Education standards are easy to drop. Watch out.
That's my $0.02.