The survey of laws, practices and size of inflows in 60 jurisdictions found Delaware coming in first, followed by Luxembourg and then Switzerland.
The Cayman Islands and the United Kingdom round out the top five.
"While the US has been jumping up and down and saying 'Aha, bad, wicked Swiss banks,' the US is doing exactly the same things as far as non-resident bank account holders," Tax Justice Network executive director Sarah Lewis said.
Switzerland has been the poster child for financial secrecy over the past year.
The United State sued Swiss global banking giant UBS AG , which paid a US$780 million ($874 million) fine to settle a lawsuit against it by the Government.
As part of the deal, UBS admitted it actively helped Americans evade US taxes.
The ranking is based on a composite of total offshore activity and measures such as whether a jurisdiction obtains beneficial ownership information about companies and the degree of cooperation in turning over requested financial information.
Delaware is attractive because it does not tax profits realised outside the state and does not require companies to be physically present, according to the Tax Justice Network.
UBS and Credit Suisse have about 200 entities in the state, according to the group.
There are nearly 700,000 active entities registered in Delaware - and about half of those publicly traded in the United States, according to the group.
Total US deposits of non-residents rose from about $1 trillion in 2001 to $2.6 trillion in 2007, according to the study.
In Luxembourg, non-resident deposits rose to $500 billion from $101 billion over the same period
In Switzerland, such deposits rose to $1.45 trillion from $103 billion during the period.
