IMO, one of the more common difficult ones is Rotala macranda.
As for 'rarest', it depends on what you're asking.
Lots of plants are rare in the US but common in Asia. Plenty of
rare crypts - even back in their native habitats.
German hybrid swords are rare in the US.


"cthulhu" wrote in message
...
Out of pure interest what is the
a) most difficult tropical plant to grow

b) the rarest ?