Interesting thread! The most comfortable seats I have found, outside of an SUV, are my 124s, both 300E and 500E! However, I have had to do some alteration.
First, my back is most comfortable when my seat is as high as possible, with my knees being as low as possible. I do that by inserting a triangular cushion in the seat in such a way that the rear of my seat sits a bit higher than the front. It works perfectly for me. I have a long trunk, and relatively short legs, although I am six ft one in. I also put the seat all the way back, and the steering wheel all the way forward.
The worst seats I have ever had was an Acura Legend coupe--way too low to the ground. I had to get out of the car every 20 minutes.

It is a totally individual preference--what works for one back doesn't work for another.

Tom
92 500E