Best BBQ on I-35 between Dallas and San Antonio?
Could you provide us with more clarification. Are you wanting something directly on the highway or are you willing to take a short detour (couple of blocks off the highway)?
Temple seems to be your best bet for BBQ - Try out Clem Mikeska's (haven't been there personally) but it has been around a long time and is part of the famed Mikeska clan of BBQ brothers. Recommendations seem to be the BBQ sirloin and the sausage.
Another more highly recommended place is Schoepf's Old Time Pit BBQ in Belton which is a short drive (5-10 mins south of Temple
I grew up in Belton and both Schoepf and Mikeska are so so at best. Millers BBQ in Belton beats both of them hands down. The only thing decent at Schoepfs is the pork chop - the briscuit is overpriced and really dry. Mikeska boils his meats before throwing on pit - its not good.
However, Las Casas Mexican right next to Clems in Temple is famous for their White-Wings and they are Excellent - definitely worth the stop.
Pecan Lodge is in Dallas and on 35 but you'll need to be there early and you'll be waiting in line for a while as its as good as any Q in the state. Lockhart BBQ is in Dallas too and is right off 35, They are run by the same people who own Kruez in Lockhart and at this point I think based on my last visit to Kruez in the city of Lockhart Id say actually better. South of Austin there is a little town called Creedmoore about 10m off 35. Theres a place there called Wilhites that is very, very good. Certainly better than Mikeskas or Schoeps.