You might as well be asking a room full of grade-schoolers who has the best mom. You'll get a lot of opinions, but the "right" answer is going to vary by individual.

These days, the gap between carriers is much smaller than it used to be, yet every carrier still has dead spots. If I told you a certain carrier has the best coverage in Austin, it wouldn't help you much if that carrier happened to have a dead spot right in your driveway.

The good news is that every carrier has money-back guarantees that allow you to try the service and cancel with no penalty. You're usually allowed up to 30 minutes of usage. So sign up, charge up the phone, and drive your normal routes and commutes with the phone on. That way you'll know for sure.