I hate to quote "studies" or "surveys" without giving particulars thereof, but in this case none were provided and I therefore cannot. The information comes from a presumably reputable reporter (Patrick Radden Keefe) writing for a publication with a sterling reputation for responsible journalism and fact checking, so it seems reliable.
In his article "Buried Secrets" (The New Yorker, July 8 and 15, 2013) about corruption in Guinea and its huge iron ore deposits, taking about how bribes are designed to be untraceable and opaque, Mr. Keefe says:
"A recent study found that the easiest country in which to establish an untraceable shell company is not a tropical banking haven but the United States."
I guess we should expect nothing less from a country that is arms merchant to the world, and spends more on the military and weaponry than the rest of the world combined.