Breeding is easy, it is the raising of fry that is difficult. In my opinion:
Hardiest and easiest to raise fry: Southern Erectus
Most time consuming and labor intensive: Dwarf (Zostrae)
Most difficult to keep healthy: lately Kelloggi have not been very hardy
Most difficult to raise fry: Ingens... however for commonly available species it would be pretty much a tie among those with pelagic fry (Kuda, Reidi, Northern Erectus, etc.)