"Great BBQ" is all about preference and we all know preferences are subjective. When it comes to taste there's really no standard for great tasting bbq because the beauty is in the eyes of the beholder.
As crazy as it sounds there are a lot of people who think Chili's baby back ribs are "great bbq". Even after they've tasted what we (the Brethren) would call "Real BBQ". A lot of it is what you've grown accustomed to eating.
For me personally I cannot understand how someone can take a perfectly good smoked slab of ribs and CAKE K.C. Masterpiece bbq sauce on it and call that "great bbq". But they do. I see it all the time. So what you're searching for is really a moving target. The "greatness" of the bbq will many times depend on you and your taste buds.