I don't necessarily have a bad back, but I have gotten tired of falling into and climbing out of sedans and coupes. Even my former Camry was too low for my liking. My CR-V is perfect: you open the door and slide right across into the seat. Our MDX is just a slight "step up" into the seat, but it's still mostly at bum level.
I think the most comfortable seat I've ever sat in is either the front seat of my old '97 Cadillac or the front seat of my parents' 2012 Jeep Wrangler. Although the Wrangler seats have Katzkin leather covers on them, so it would stand to reason that they'd be plusher than usual. I didn't care for either of my Toyota seats ('07 Corolla, '11 Camry) because the seat bottom, the cushion, was very short and offered little thigh support.
For overall comfort (and for other reasons), I've gotten to preferring SUVs. They're not too short and not too tall, and seem to do everything well that I want a vehicle to do.