What snake should I get?

[TRD]

1st of all, get a snake that you like, that YOU find interesting. If you get a snake you don't like, you're less likely to take interest in it. Snakes live a long time and there's nothing to win by either you or the snake if keeping the snake becomes "dropping in a mouse every week and cleaning it every month". It will add no value to either your or the animal's live.

There are many more snakes that remain small, and generally stay calm. Why is it only sandboa, ball python, and cornsnake?

1. Sandboa you will barely see, if ever. They don't take well to handling. unless you REALLY like the species, I would advise against it.
2. Ball Python people generally see as a "pet rock" however it only forms this "rock" as a defensive posture. It's not fun for the animal. They are pretty shy.
3. Of the species you listed probably the cornsnake would fit your bill best. Very easy going snake, at least in general.

So what are you looking for. Snake to watch? Snake to handle? Do you want to build a sub tropical or desert vivarium? What space do you have? Do you want to see your snake, or you don't mind to see him sometimes? Which food items can I get easy access to? etc.