This kind of depends on you. Do you want a larger species or a smaller? Are you set on kingsnakes or just colubrids in general?

In my opinion, there really is no better first choice for a snake than a california kingsnake. All the ones I have dealt with have never tried to bite me (though if you aren't careful during feeding time, you are liable to get tagged if you're holding the mouse by hand), they always eat when food is offered, are a very manageable size, and are extremely easy to care for. Even if husbandry is sub-optimal, they'll still take anything you throw in there.

There are also a bazillion morphs for them, much like the ball python (all black california kingsnakes do exist and look pretty much identical to an MBK). Perhaps not quite as wide a variety, but there's a lot. They are the easiest pet snake I have ever taken care of.

If you are open to other colubrids as well, there are some spectacular choices to choose from. Tiger ratsnakes, San Luis Potosi kingsnakes (so called "mex-mex"), California red-sided garter snake, any Drymarchon species, Blue Phase Kunashir Island Ratsnakes, Mandarin rat snakes....the list just goes on