The big 4 for BBQ are Memphis, Kansas City, North Carolina, and Texas.
My biggest disappointment has been Texas BBQ. While in Austin a couple of years ago I sampled from three different restaurants and they were all good but none of them overwhelmed me. Besides looking online- the best thing to do when looking for good BBQ is to just ask a local - they usually know better than anybody.
These are the 4 BBQ joints that have really stood out since I started this quest.

Lannie's Bar-B-Q Spot Selma, AL 2004

Interstate Bar-B-Q Memphis, TN 2008
Arthur Bryants BBQ Kansas City, MO 2007

Payne's BBQ Memphis, TN 2005