For me it is the Green Knight.
I keep wondering why people keep upvoting the "flower power" paladin...?
I can't speak for everyone, but I'll tell you why I'm upvoting the Oath of Ancients: it's the only one I've played.
Which means that of all the paladins, it stood out as interesting enough for me to actually consider. It had the best flavor imo...a paladin who swore an oath to the forests of his home, a protector of the earth and the wild places. Every other paladin is another edgelord anti-hero, or another "battlepriest-plus-a-gimmick." Been there, done that.
So like I said in another post, I rolled up an Oath of Ancients paladin, choosing Shifter as my race, and picking the Outlander background. I wanted him to be completely different from the same old Churchy Fighter that everyone else plays, so I decided that my paladin and his whole clan practiced ancestor-worship..."the gods" were actually his distant relatives. His clan believed they all descended from a "great wolf" long ago, so my DM suggested that his great-great-great-grandma was probably a werewolf and was therefore the seat of the clan's power.
Anyway, it was a fun character to play.