A lot of "cleaner" catfish are supposed to be hard to breed. for instance otto's and some cories. I would think that a Black ghost would be very hard to breed! Also, Ive herd cardinal and neon tetras are very hard.
With my fish, its not the breeding part(spawning) that is hard, its the keeping the fry alive and not having the eggs eaten.-so frustrating watching two beautiful discus spawn and then the tankmates pushing them away and eating the eggs!