CCNP, CCNA, CCIE are in a different category than CISSP. CISSP is an IT security cert that has more to do with policy and standards than administering anything. I know people that have taken all of them and said that CISSP was the most difficult, but there's also people I know that have CISSP and I wouldn't have guessed them to be all that intelligent. What's difficult for some, may not be for others.