The countries with the lowest rates of corruption, according to Transparency International’s Corruption Perceptions Index for 2011 (the only source of its kind).
Other countries in the top 10 include Canada, Australia, Norway, the Netherlands, and Switzerland. The US ranks 24. Note that countries with big governments can be found all across the spectrum, from cleanest to most corrupt. Many of the world’s most secular societies also rank highly as well.
I think Canada and Australia are particularly impressive in their performance, since they’re far more populous, diverse, and geographically large than the others that rank highly. The trend seems to be that countries which homogeneous, small, and demographically compact tend to be less corrupt. But of course, it’s far more complex than that, and many other factors contribute.