I have found this is very seat dependent. I am very heavy and the Emirates A380 seat is by far the most comfortable for 10-14+ hour flights. Most planes are OK

I have found this is very seat dependent. I am very heavy and the Emirates A380 seat is by far the most comfortable for 10-14+ hour flights. Most planes are OK but the new Qantas 787 Economy seat for some reason is not. The new Qantas 787 Premium Economy seat however is amazing. Among other things the whole seat tilts back slightly (including the base) and it has an adjustable height foot rest. It was a dream to sleep in and I'd consider paying for it (every flight I take is usually a 20+ hour trip, so..)