So even though I prefer all three Carolina styles (four if you count the very rare "green"), they won't show up on my list of top 20 BBQ cities as none of the great examples are in "Cities". Because while I enjoy visiting, Shelby, Lexington, Goldsboro and Owensboro are not real cities just pleasant small towns.
I would rank the mid and large sized cities thusly
1) Memphis
2) Birmingham
3) Kansas City
4) St Louis
5) Santa Maria/Santa Barbara
6) Nashville
7) NYC
8) Santa Fe
9) Austin
10) Albuquerque
11) Tucson
12) Phoenix
13) Charleston
14) Richmond
15) Baltimore
16) Montreal
17) Savannah
18) Lafayette
19) Tampa
20) Atlanta
Near misses would be Louisville, Dallas, Cleveland, Houston, LA, Orlando, Jacksonville, New Orleans, Portland Oregon, Portland Maine, Amarillo, Raleigh, Durham, and Louisville in no particular order.
And personally I think there's huge dropoffs between 5 and 6, between 12 and 13 and between 16 and 17. In other words, I'd rate the top five true vacationworthy/focus your time around BBQ and give them a 10/10. 6th through 12th would get 8s, 13th through 17th would get 7 and 6s, and 17-20 plus my other mentions would get 5s.