The airport is sort of in a BBQ no man's land. If you've a rental car, I'd personally recommend just driving down to Arthur Bryants or Joe's to get the real deal. Smokebox is probably fine (never been) but if you don't get here often then I'd recommend driving the extra 10-15 minutes for a known place.
-Joe's KC
-Arthur Bryant's
-Jack Stack
-Q39 (newer place but already in the top 10)
-Char Bar
-Gates
-Woodyard
-Rosedale
-Slaps