Volvos are always good on the seat comfort and support front, while quite a few Opels have seats that are approved by the German back-pain association. Nissan's current-generation Qashqai has seats partially designed by NASA and they're pretty good too. 

 

As a fellow bad back sufferer, I can tell you that most cars are pretty decent when it comes to supportive seats but an occasional one can just sit you wrong in the car and triggers some pain. Your best bet is to try some out and make sure you can get comfortable before you buy. Take as long a test drive as possible so that you have a chance to properly assess the seat comfort.