There is no definitive answer to this question as it depends on a number of factors, including an individual's qualifications and experience. However, some federal agencies are generally considered to be more difficult to get into than others. These include the Central Intelligence Agency (CIA), Federal Bureau of Investigation (FBI), and National Security Agency (NSA).
The CIA is responsible for gathering intelligence and conducting covert operations overseas. As such, it requires its employees to have a high level of security clearance and undergo extensive background checks. The FBI is the primary law enforcement agency in the United States, investigating crimes ranging from terrorism to public corruption. Like the CIA, applicants must have a top secret security clearance and pass a rigorous background check. The NSA is responsible for collecting foreign intelligence through electronic surveillance and code decryption. It also has strict requirements for employment, including US citizenship and a top secret security clearance.