In the Line of Duty (1-2). Brubaker & Lark offer a great, shocking start to the series, then show-off all of its great elements: an intriguing, real group of detectives; fearsome villains who are horrific for ordinary people; and an adversarial relationship with the Bat. The concept is great, as is the writing [8/10]. 
 Motive (3-5). This second arc doesn't have the same emotional resonance as the first one, but it's a brilliant mystery, carefully laid out, with realistic-feeling police work. It also does a great job of showing off an ever-increasing cast of characters [9/10].