I certainly enjoy the original 3 books the most out of the OId Kingdom series, but Clariel and Goldenhand are also very good reads :) As is the short story he did about Nicholas. I forget the name, something about a creature? It's in his "Over the Wall" collection anyway. My second favourite...
Welcome! Another Garth Nix fan here, particularly the Old Kingdom books :) I hope your first day at work goes well when it comes! I'm another "couldn't be a SAHM forever" person. Hats off to those who can do it, but it's definitely not for me.