Live bearers are generally the simplest fish to breed, because they pretty much do so whether you want them to or not. Guppies have been bred in so many pretty patterns and colours, even the females have colour now. Endlers are fun though. Closely related to guppies but quite a bit smaller. Females are a nickel silver shade, only the males have bright colours. The wild form ones are rather variable, but there are several colour strains, such as Black Bar, or Neon Reds. There are also Endler/Guppy hybrids, which are usually a larger fish. Both are prolific, as are most live bearing fish. Platies are another you might consider, though they're larger, they also come in some beautiful bright colours and are hardy, easy fish to keep. They all like hard, alkaline water conditions best.
Clean water and regular maintenance are your best pals for any breeding efforts. Feeding some frozen or live food is always a good idea too, providing better nutrition for breeding fish.
