From my (limited with livebearers) experience, I have had most luck with swordtails. Ihad a pregnant female guppy, but she died before having the fry. I have had two batches of swordtail fry, one of which is now about half-grown and the other (born on Easter!!!!) got eaten by the mother
I didn't even want to breed fish, but yet still managed to breed guppies. I've had three groups of fry so far with at least 2 more seemingly on the way. I haven't tried swords, mollies or platies. I've had bad luck with swords though. I ordered a Hifin Lyretail sword from Dr. Fosters and Smith and it died 3 days later. I also bought a velvet wag sword from my LFS and he died just a couple days later. Can't quite figure out why seeing as how my water is near perfect.