I just got back from a trip to Hattiesburg, MS, and I can now recommend a place:
Lethea's Bar-B-Que Inn.
No muss, no fuss, simple and delicious menu. All the meat is slow smoked/cooked, and the homemade sauce is amazing! It's not too spicy, not too sweet, but fits the meat perfectly (and they bring a little pitcher to your table, piping hot, so you can sauce to taste). Also, try their cole slaw. Best slaw sauce I've ever had.