Which of the following categories of hackers poses the greatest threat?

According to the authors, hackers fall in these categories, in increasing threat order: security experts, students, underemployed adults, criminal hackers, corporate spies and disgruntled employees.

Disgruntled employees are the most dangerous security problem of all because they are most likely to have a good knowledge of the organization's IT systems and security measures.

Source: STREBE, Matthew and PERKINS, Charles, Firewalls 24seven, Sybex 2000, Chapter 2: Hackers.