Each person is made differently - for me, the absolute best position for sitting for a long time, comfort wise, is leaning forward on to my knees (takes the pressure off two of my discs. However, on most economy seats, this is impossible if the person in front is reclined, so my second choice is reclined back with a lumbar roll. My least comfortable, and will hurt during and after the flight, is bolt upright. This doesn't just apply to planes, but most places if I have to sit still for extended times.