Certainly not the rarest, since there are one or two out there, but definately among the rarest things out there.
Voidwatch Uptala's "Murasamemaru". I've never met anyone who has one, ever. But they're supposed to have dropped atleast once at one point.
Same for Voidwatch Qilin's "Coruscanti". Only one or two people in the entire world of all servers seem to have one. I'm personally already 0/300+ on it, but everyone else who ever done Qilin is probably a good 0/1xx on it as well.
After that, any of the special Voidwatch Armors. Only just the other day got a Mextli's Harness, never seen any of the others. There's a reason the inferior crafted version sells for a few mil, and the still inferior crafted +1 version for 8-9 mil. If it was easy to just go out there and get this one, people would do that.
Semi rare, the Ogier's, Athos's, etc.
None of these are as rare as previously mentioned items. A search of FFXIAH will reveal that, but then again a casual stroll through my own server's Port Jeuno did this weekend.
Also 0/300 is starting to approach the average of most VW rare drops.
Rarest is probably Laurel Crown. Fortunately I had the chance to Ballista a few times against one of the winners, Barlog, when I was still on Siren.