Transparency International ranks the United States 19th on a least that ranks nations from least to most corrupt receiving a score of 73 out of 100 on their scale (confidence interval 66 to 79). Puerto Rico is ranked separately in 33rd place with a score of 63 out of 100.
Denmark, Finland and New Zealand were tied for least corrupt nation with scored of 90 out of 100 on their scale. Other nations less corrupt than the United States, in order, were Sweden, Singapore, Australia, Switzerland, Norway, Canada, the Netherlands, Iceland, Luxembourg, Germany, Hong Kong, Barbados, Belgium, Japan, and the United Kingdom.
For what it is worth, I probably wouldn't rank all of those nations as less corrupt than the United States, although there are probably a few nations not listed which I might rank higher (I considered France before I recalled that it accepts the practice of kidnapping senior managers in the course of union-management negotiations, and Austria, before I recalled that they recently gutted their constitution to favor the ruling party). Three of those nations ranked less corrupt than the U.S. (Switzerland, Luxembourg and the Barbados) are known for involvement in confidential off shore banking. Iceland recently defaulted on its national debt. Singapore is an authoritarian city-state known for cracking down on political dissent.
Japan is a mixed bag - in some respects it is far less corrupt than the United States (e.g. people are far more likely to admit wrongdoing and accept fault when appropriate), and in other respects it is pervasively more corrupt than the U.S. (e.g. my perception is that there is far more nepotism and far more abuse of authority by those in leadership roles, and that there are far more undocumented and undisclosed business arrangements, than in the United States).
So, on the whole, I'd have to say that the United States was somewhat underrated in the survey. Then again, the United States is a big country and some places in it are less corrupt than others. North Dakota, for example, is far less corrupt than Mississippi. Connecticut, similarly, is far less corrupt than Texas. Colorado is probably less corrupt than the average U.S. state.
Greece was one of eight countries tied for 94th place and the most corrupt in the E.U. with a score of 36 out of 100.
Afghanistan, North Korea and Somolia were tried for last place (174th) with scored of 8 out of 100.