Hi Keith, I've just walked around my garden and at this moment in time, the darkest green is my Westonbirt orange. All could and will be different in a few days. They change so much. Here is a photo taken a few minutes ago.
'Shishigashira' is a good nomination from the palmatum species. More broadly, A. fabri is a very dark green all year and especially in summer after the new growth fades from red. A. sinense is very dark too, and of course A. pubinerve begins the season as a shiny brown, fading into a dark green. Sorry no time to search for pictures just now.