Convicts
Got a pair for 3 weeks and they've already bred twice.
First time was unsuccessful, female laid eggs again 2 days ago and now I have babies!!!
Live breeders i agree great for feeder fish and you dont have to quarentin em as they are home bread. Severums and convicts are easy as long as you have a dedicated pair.
Gambusia, aka mosquito fish. Just try to stop them. If you throw them in a bucket of acidic/alkaline/hot/cold/clean/murky water and fed them 20 year old goldfish flakes, you'll have fry.
Just be careful and never, ever take a chance of letting any escape into a local body of water. They can and will take over just about any habitable body of water.