I vote Endlers, Guppies, or Platies. Swordtails need more room, and the males can get pretty aggressive, sometimes killing each other. They need a lot more room. I know a few people have different experience with mollies, but most are sickly in fresh water- you have to have perfect conditions to keep them healthy under those circumstances.
Some livebearers, like Goodeids and halfbeaks are very difficult to breed. At a
couple recent auctions, people have said "these fish will give you cichlid breeders a real challenge!"