By rare, I mean what anime have you watched that you think has the least number of other people here who have watched it (or even heard about it). Of course rare anime includes both good and bad, but luckly all the rare anime I've seen is good. Here are my top 3 (that I can remember):
Photon, a short 4-6 episode series about a boy who is incredibly strong and saves a very
intergalactic female warrior/princess (and man did she have boobs!)
It was absolutely hilarious.
Infinite Ryvius could be my favorite anime of all time. Think of the novel "Lord of the Flies", but make the characters more interesting, the plot more outstanding, and the setting in outer-space. I absolutely ate this series up
Blue Seed was alright, but nothing special. Definitely worth the $40 I shelled out for the whole series
Plus the omake theater at the end was hilarious
These are just series that I THINK are rare. I don't know, they could be totally popular, but I've never heard anyone mention them in the last 15+ or so months since I registered here. (I do know, however, that all 3 of those series are licensed)