Opie's BBQ in Spicewood, just west of Austin. Other than that, usually anywhere that isnt a chain store and the parking lot is full, is good with me.
When looking for good BBQ in a new area, I always try to find the littlest hole-in-the-wall place as they usually have the best BBQ.
Thelma's BBQ near downtown Houston... It's an old beat up shack with no A/C and you have to wait 1hr for your food because of the line, but it's worth the wait. If you don't want to wait, Thelma will come out herself and tell your azz to get steppin'.