The last in the series, and suitably explosive and bloody. Reflecting on how I came to work my way through the lot despite not having any particular taste for ultra-violence nor for the grit that characterises the series, and particularly while having some serious reservations about the politics / ideological standpoints of the books, I think there are (lemme count) three main reasons.
First, they're written and drawn with a helluva sense of pace - each one flies by (I read this last in a single sitting in the Fitzroy library a couple of weekends ago). Second, while Preacher isn't long on character development as such, Ennis excels in deepening our understanding of the characters as he goes along. And third, there's the mythos invoked by the books, and ultimately that's what gets me (even if, on closer inspection, it possibly turns out to be bound up rather closely with the essentially conservative viewpoint espoused by the books that troubles me).
[1], [2], [3], [4], [5], [6 & 7], [8].