LDA105 is the rarest pseuda, no question about that
The rarest plecos in terms of pieces available in the hobby are L250 and neblinichthys pilosus (the latter of which no living piece exists in captivity, to my knowledge).
One look at the Cat-e-log on Planet Catfish and you get a sense for how many undescribed species there are. Take into account the political situations in many of these countries, the remoteness of some of these habitats, and imagine how many are yet to be discovered.