well i can tell you the three rarest (in the hobby):
1) P. bichir bichir. it was available only since around 2002 or so, BUT only in japan as far as i know, and only in limited numbers for these last few years. But recently the japanese succeeded in breeding them, so their numbers in japan will increase. However still unavailable elsewhere.
2) P. ansorgii. first became available in 2006 (first in japan, then in US). numbers available is increasing, but still small overall. BUT consider this: ansorgii was described in 1910, and for nearly a century, only three or so museum specimens existed worldwide, no live specimen seen in the hobby or scientific community until now.
3) P. teugelsi. new species. described in 2004, which was about the time when they became known to the hobby. first specimen imported into japan in 2006. and in 2007, other countries (US, UK, Germany) have limited number of them available.