I hate to admit, but of all cars we have had, a Dodge Caravan has had the best seats. It is like a small recliner. Now the rest of the car has not been that stellar.
At the end of the day, as mentioned, you have to test quite a few. Ideally rent the model that you like for a week and see if you can put up with it. I have a bad back, the compromise car after the Minivan has been a Honda CRV, it gets better gas mileage and still decent seat comfort-not equal to the Caravan though.