The study, in which Transparency International looked at 13 independent surveys, gave New Zealand a 9.4 score out of a possible 10. Denmark was second at 9.3 with Singapore and Sweden each at 9.2 and Switzerland at 9.0
Somalia scored 1.1 and is at the end of the 180-country list. Also near the bottom were Afghanistan (1.3), Myanmar (1.4) and Sudan and Iraq (both 1.5).
The United States ranked 19th at 7.5, just behind Japan and the United Kingdom (tied with 7.7 ratings). Canada (8.7) was tied for eighth. The other U.S. neighbor, Mexico (3.3), was 89th.
Only 51 countries beat the index's midpoint of 5.0.
Transparency International said the highest scores "reflect political stability, long-established conflict of interest regulations and solid, functioning public institutions." Countries at the bottom were generally "plagued by long-standing conflicts, which have torn apart their governance infrastructure."
"Overall results in the 2009 index are of great concern because corruption continues to lurk where opacity rules, where institutions still need strengthening and where governments have not implemented anti-corruption legal frameworks," Transparency International said.