Well, I'm liking night watch, it doesn't have as much light humor as the others.. more of a solid situation/ resolution with some character studies.

I think Thief of time (#26?) is good starter , too, especially if you dig time-travel yarns.

I especially like the ones that focus on the Watch, but I think I've actively enjoyed all of 'em except for "moving pictures"


(probably my favorite group of charaters in the discworld universe - Other sinclude some hedge-witches, a univeristy of wizards, the incarnation of death (who is actually rather nice), and others single-shots.)