Probably Platies...Platies are livebearing fish (don't lay eggs) and the females are pretty easy to breed. Get the breeding net and try and net as many of the babies (fry) as you can and place them in the net. This is to keep them safe from the other fish. Or, you can go opposite and net the parents but the net is really too small an area for them for longer than maybe a few days.
Use small food as the mouths of these fry are tiny. Well ground flake food, someone suggested the Hikari version, works. The problem is that it is very easy to overfeed and the net won't allow the food particles to get sucked into the filter.
The fry can be released back into the main tank once they grow a bit. Upon release, have a bunch of fine leafed plants in the tank in any case to allow them to hide if necessary. Eventually, they will get too big for the other fish to consider them as food vice other fish