One of the rarest boids in the world is the 'Round island boa' (Casarea dussumieri ) from Mauritus.
There is another thought to be rarer, but that is now thought that it might well be extinct, that was the Cropan's boa (Corallua cropani), there are only now 3 specimens of them, and they are dead in Jars, who knows someone might be lucky enough to find one and prove everyone wrong about their status.
As was discussed on another tread previously, the Rough Scaled python (M.carinata) isn't as rare as you think, just isolated and hard to find, it's only listed on Cites as appendix 2, which means they don't consider it in danger of becoming extinct or threatened. All Boids are appendix 2, apart from the ones on appendix 1, and no Aussie Boids are on that list as yet !!
NCHERPS