Not sure how rare it is per se but it's one I hardly ever see available, Gorgonia ventilana. It's probably my favourite in my tank though:
It's a Caribbean species which is why you almost never see it as collection of anything from there is so heavily regulated (probably rightly so). But it's too bad because it's a super hardy photosynthetic gorg that loves the same flow and light conditions as SPS.
One coral I wish I could have (but never will) is Diploria labyrinthiformis or maybe Diploria strigosa. Also Caribbean endemics, as far as I can tell, basically totally illegal to possess, period.
I'll have to content myself with photos.
My obsession with Diploria made me try to content myself with the similar Pacific Platygyra coral, but that thing is miserably aggressive and takes down anything placed within 6" of it. I just noticed tonight it took out one of my favourite Acan's, literally gone overnight. Tempted to put the Platygyra up for sale, as neat as it is I'm tired of losing $40 frags because it got all territorial and sent out 8" sweepers to destroy them.