Guppies and goldfish are some of the fast reproducing species. Locally, we have Tilapia as one of the fast reproducing fish. Maybe we can tell by their price. They are normally cheaper if they are quick to reproduce.
Definitely, guppies and goldfish are among the top easiest to breed and fastest to reproduce. But may I share my experience where I place swordtails and platies in one set up and I observed that they can actually interbreed. They easily reproduced. So if you want to have pure breeds, I suggest that you should separate those two breeds. Anyway, Swordtails can also reproduce fast and easy.