Wiley's BBQ on Whitemarsh island I would say is the best. If you're downtown, there's 3 joints: Angel's on Oglethorpe Lane (kind of mushy for my taste) Blowin' Smoke on MLK Jr. Blvd (also a little mushy) and Barne's on Whitaker (not much flavor on the pork, but nicely pulled).
If you're feeling adventurous, there's a place called the Rib Hut in Garden City (about 2 miles east of downtown on Bay Street) that I've heard is real good.