It should be the 2013 Kindle Paperwhite since it has the latest e-ink screen (carta compared to the rest still using pearl). I remember someone posting a link at some point showing how black the text and how light the screen was on the latest ereaders and I *think* the PW did come out on top, but I can't find the post nor the link now.....
Besides, its possibly subjective anyway, so it might be worth your while looking at some comparison videos on ebay that compare the lighted readers, just to see what you think yourself.
For example:
Paperwhite (2013) v Kobo Glo
Paperwhite v Kobo Aura
Paperwhite v Kobo Aura
New Kobo Aura vs Kobo Aura HD Comparison
Kobo Glo v Kobo Aura