Transparency International's 2007 Corruption Perceptions Index scored 180 countries based on the degree to which corruption is perceived among public officials and politicians.
Burma and Somalia received the lowest score of 1.4 out of 10.
Denmark, Finland and New Zealand were ranked the least corrupt—each scoring 9.4.
"Countries torn apart by conflict pay a huge toll in their capacity to govern," the agency's international chairman Huguette Labelle said in a statement. "With public institutions crippled or nonexistent, mercenary individuals help themselves to public resources and corruption thrives."
Western governments have accused Burma's junta—which seized power in 1988—of turning what was once a jewel of Southeast Asia into one of its most miserable places through repression, mismanagement and corruption.
Burma's business elite thrive by serving the generals, while many in the country go without regular food and electricity, the top US diplomat in Burma, Shari Villarosa, told reporters earlier this year.
Somalia has been ravaged by violence and anarchy since warlords overthrew dictator Mohamed Siad Barre in 1991 and then turned on one another. The current, UN-backed government was formed in 2004, but has struggled to assert any real control.
Faring the best in the survey were Denmark, Finland and New Zealand, in a first-place tie with each scoring of 9.4.
The agency's scale is based on the perceptions of the degree of corruption by businesspeople and country analysts. Countries are ranked out of 10, and any score below 5 indicates "serious" perceived levels of corruption, while scores below 3 reflect "rampant" corruption, the agency said.