For a first breeding effort, I would generally recommend egg layers. Most egg laying colubrids are easier to breed.
African Housesnakes would be the easiest to breed and deal with. All you need is a male and a female, they don't need to be cooled and the babies usually take pinkies right away (the big dark greenish colored Housesnakes have bigger babies generally).
Cornsnakes are also pretty easy, as are Cal Kings and Mexican Black Kings (and you already have half a pair for either of those two taxa).
Some live bearing snakes (Gartersnakes, for example) are pretty easy, but live bearing boids tend to be a little bit more work (it is hard to get to observe mating and harder still to predict if and when birth will follow).
Basically, you should use two criteria when deciding to breed snakes -
1. What species do I like? Don't bother getting something just for "breeding practice". Find a species you are interested in and breed those.
2. What species would be easy to get rid of as babies? Make sure the babies are easy to start feeding and that someone would want them. While you may really like some obscure african colubrid like Psammophis, getting someone else to take some of the babies off your hands may be quite difficult.
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Chris Harrison