I agree with dan518, N. ramsdeni, its the only one never available(or hardly available) in the states because of U.S./Cuban politics.
For me, the 2nd rarest is Isthmoheros tuyrense, and is one of the reasons I chose Panama as a place to move to, to collect my own.
It comes from large rivers on the edge of the Darien that are remote, and hard to negotiate in any, but the dry season.
But is also found in Lake Bayano.
Now that I've acquired another tank, it is next on my list, along with Darienheros calobrensis.
I believe tuyrense is also rare, because it isn't full of gaudy colors, and gets quite large, any tank under 6ft, would only be a grow out tank for juvies of this large vegetarian cichlid.