If you're going to be downtown or on the north side, you have to try Fat Willie's, on Schubert near Western. It's just south of the intersection of Western & Diversey, an easy cab ride from downtown.
They do real low-and-slow smoked barbecue, not an imitation. Their pulled pork sandwich is huge, and excellent. The ribs are great, and I've heard their brisket is too, but I can't seem to order anything but the pulled pork.