Mostly various Carassius Auratus (goldfish); all different types, and Shubunkins.
In this new system, I use Trout (Rainbow & Brook); both coldwater fish. Trout are restricted in Maine, and I had to acquire a permit to stalk them.
The most common type of fish used is Talapia, but they are restricted in Maine because they too, can survive through the winters here. I have no problem getting the license for them, but I'm limited with where I can get them, and my location is very far away. Maine is a very big state, and the closest Talapia Hatchery is 5 hours away. A Trout hatchery, on the other hand, is 20mins from me. Koi is also a very common fish to use.
The easiest fish to work with would be Koi, because they're like a tank in the water; meaning they're hard to kill. They eat anything and can survive in horrible conditions. Shubunkins and Goldfish are a close second, but Shubunkins grow faster.
People use Talapia because they're another fish that can survive in cold and warm water, they're also a hardy fish, not quite as much as Koi, Shubunkins, and Goldfish, but still a good choice.
You can also eat Talapia.