Probably the Gateless Gate, the Eiichi Shimomissé translation. I'm actually a Discordian, but I find the Principia best for introduction. It devotes a lot of space to silly rules you're supposed to violate and other introductory concepts and practices. And Illuminatus! is plagued by a masculine confidence and aggression that both the writers and Hagbard were aware of and tried to minimize. The Gateless Gate is, to me, much better for staying deep in the untethered state of pure Discordian existence. It talks a little much of patriarchs, but it's not thematically essential. And it isn't rooted in and doesn't reference modern western theology and philosophy like the Principia because it was never intended to stand in contrast to or lead people out of modern western theology. Both the Principia and Illuminatus! reference it in some way because secular zen is important to the development of Discordianism. Maybe no book has ever changed my life as much as Illuminatus! but the Mumonkan is one of my primary tools for staying rooted in this way of being. It's with me all the time. One of the first things I do when I get a new phone is make sure my Mumonkan made it over or go download it again. I read it whenever I'm feeling lost or confused and uncertain about a decision or life change. It always leads me back to me.