Agh! Just one? Don’t make me choose between Nightwing and Birds of Prey!
In fact, I’m going to be cheap and just say Detective Comics, in general.
In all seriousness, I love me some '90s/early-'00s Bat-Books. They seemed to strike a good balance between the usual Batman-ish darkness and actually being fun to read. If I have to pick something, I’ll reluctantly go with Chuck Dixon’s Nightwing run. Dick gets to be a cool dude just doing his own Nightwing thing independent of both Batman and the Titans. Great supporting cast, cool lesser-known villains, a bit of drama, a bit of action, and a sense of humor. The art’s not ideal, though. Scott McDaniel’s art makes it impossible to tell what’s going on, and when Greg Land takes over, it’s frustrating because the guy could actually draw pretty well, so it’s annoying seeing his early work (like this series) and knowing he eventually resorted to incessant tracing. That said, unless the art is really good I tend to pay more attention to the writing, so it didn’t really detract from the book for me personally. It’s just something to know going into it.