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I've said this for as long as the A350's and A339's have been in service. That old 'coffin seat' as many of the online keyboard warriors have sadly labeled it is by far the best seat when in bed mode. It's wider at the shoulders and flatter than all the other seats.
The worst seat in bed mode by far is the A350 seat as that one barely even goes fully flat. The only saving grace for that seat is if you manage to grab one of the bulkhead seats in row 1 as those have an extra cushioning bit near your feet making it wider.
Both the A350 and A339 seats are a pain to get out of when in any sort of reclined position.