Hard to say, Clownfish, seahorses and cardinals breed most readily from what i've seen/read. Certain types of gobies (especially neon gobies) and blennies are known to mate quite a bit as well. Dwarf Seahorses or clown gobies are my next breeding project I think
Clownfish are one the easiest to raise once you've got it down. Most people generally get around 40-60 surviving each clutch.
Clown gobies quite often breed once you've got a mated pair. Only problem is they like to lay their eggs on coral (especially sps), and during the process they like to destroy the coral a little bit I'm still in the process of reading about them.