Livebearers are great to breed, but you end up with more than you want haha you could also try a pair of convict cichlids if you have a good size tank. They say that all they need is a "tank and water" to breed so are quite easy.
Platies are actually some of the slower breeding fish on the list so far. Why do you need fast breeding fish? Do you need them to breed fast, or do you need to produce mass quantities in a hurry? if the latter, then look at danios instead of livebearers. Livebearers might give you 30 or so fry per batch, but danios will give you hundreds at a time.