I've kept all and like all. When you say "best market" are you looking to breed high end morphs? Then the answer is Western hognose, all day and twice on Thursday.
"Better snakes to keep and sell?" Well, they are all easy to keep, at least once babies learn to eat*. <-note asterisk
"Sell?" I would say hogs have by far the best market, distantly followed by sand boas, and garters are under-appreciated. I like mouse eaters in dry cages, so garters aren't something I keep any longer, though I have seen some huge ones on a largely mouse diet -there was a guy locally with lots of garter species in our herp club, and many were beautiful and larger than I'd ever seen garters get.
Sand boas are pretty worms, and there are some neat mutations coming out. I can see getting hooked on those, their little beady eyes and snub faces are awesome in any color. Google a striped Kenyan some time! Just wicked looking. I wish they were a little bigger and more outgoing.
Hogs fast (go off of feed), not all of them, and not at any particular time of year, and not every year; but they go off of feed for weeks and months at a time sometimes. They pretty much always start eating again one day, though. Males are worse for this. That personality trait isn't for the worrisome. Also, hognose can give a nasty bite if you are careless at feeding time. Females are worse for this. They will not bite defensively, but are not good snakes to feed in a tub, IMO. For about an hour after eating, many of them will chew on everything. *Remember that little asterisk form earlier? Well, as ravenous as some adult hogs are, babies can be a PITA. Become a Jedi master at getting baby corns started before your first clutch a clutch of baby hogs and you'll be okay. Hopefully.
"What is the cheapest investment?" Are you asking what the least expensive of these snakes is? A more common garter, for sure. If you are buying inexpensive stock, your "investment" isn't going to pay for your mouse bills, though -barring a miracle stroke of reverse Murphy luck. You simply will not be buying animals of which will make babies you can reliably sell to cover feed/bedding/occasional vet bills. I wouldn't look at buying any commonly kept species of snake as an "investment."
I *do* have several snakes I consider investments, and I do breed, but my more common mutations are never bred anymore. As much as I love Okeetees, there is no reason for me to make any more of them. My "investment" snakes aren't really even anything I plan to profit on, but if they pay for themselves and allow me to trade for some more neat stuff, then it keeps this hobby rolling for me.
