What do you plan to do with the offspring? That is really the question.
Livebearers are the easiest. However, they're easy to the point that they can overpopulate a tank and are hard to get rid of (everybody has livebearer fry). You can have a batch every month from each female so their populations grow VERY fast.
Convict Cichlids are also easy, but they're also hard to get rid of.