The fraud rate isn't known, but it doesn't have the lowest percentage of improper payments.
An August 2014 GAO report SNAP Enhanced Detection Tools and Reporting Could Improve Efforts to Combat Recipient Fraud notes that the fraud rate is unknown, but notes an improper payment rate of 3.4% for fiscal year 2013.
According to a September 2012 U.S. Department of Agriculture Office of Inspector General (USDA OIG) report, the magnitude of program abuse due to recipient fraud is unknown because states do not have uniform ways of compiling the data that would provide such information. Therefore, in the report, the USDA OIG recommended that FNS determine the feasibility of creating a uniform methodology for states to calculate their recipient fraud rate.8 As FNS seeks to address this recommendation, it is legally required to monitor its potential improper payments of SNAP benefits. The agency estimated an improper payment or error rate of the program at 3.4 percent, which represented an estimated $2.6 billion in wrongful payments, in fiscal year 2013.9 The percentage represents benefits distributed in error due to administrative as well as recipient errors, not all of which can be attributed to fraud. However, due to the large dollar amount involved in improper payments, the Office of Management and Budget (OMB) has placed SNAP on its list of high-error programs.
This ranks SNAP 11th out of 13 high-error programs (i.e. those programs that have +$750 million in improper payments) behind PELL grants and Retirement, Survivors, and Disability Insurance (RSDI)