there are still hundreds of L numbers that have never been bred in captivity, so impossible to say which is the hardest of all.
its also impossible to say which L number is rarest in the wild as no population count has ever been done.
if you wanted to get a species that would be ultra desirable, and extremely hard to breed at home, i'd say that the blue eyed plec would be the one to aim for at the mo.
the size of tank needed, the lack in info for breeding, the expense and scarcity of adults will all be factors that rule most people out of trying them