It doesn't have to be the number one most fascinating thing, but something that you enjoy telling people (who aren't necessarily psychologists), or something that strikes you as truly remarkable.
For me, I think "moral dumbfounding" is interesting: the way many of our moral judgements are automatic and visceral--such as why we think incest is wrong--and we then attempt to rationalise our belief. I like telling (open minded) people the scenario Haidt uses about the brother and sister.
I'll sneak one more in: humans are born premature; we "should" be born at 22 months, but owing to bipedalism --and what it did to the human pelvis (narrowed the birth canal essentially)--and humans' extra-big brains, evolution came to a tradeoff and we're born at 9 months, very helpless compared to the offspring of our ape cousins.