I concur with Sweet Baby Rays being wonderful. I must try Bone Suckin'. (hehe)
My two very favorites, however, aren't available throughout most of the US. Montgomery Inn in Cincy sells their sauce in area grocery stores--you can order it online, but I realize that's a lot of effort/money.....
Montgomery Inn Barbecue Sauce
My very very favorite is from the Fancy Farm picnic in Graves County, Kentucky (which is, pound for pound of food, the largest picnic in the U.S.) They have a tangy, vinegary sauce that I adore. It's also insanely cheap....but you can only get it at the picnic once a year, so I buy 2-3 bottles and try to make them last.