I'd recommend any of the Bci (common redtails or insular varieties), Dumerils boas, sand boas, Solomon Island ground boas (captive bred only and at least a yearling to avoid most of the feeding problems), Colombian rainbow boas, Brazilian rainbow boas (if you're willing to commit to the humidity and other husbandry issues), or maybe the rubber boas (not very familiar with them, but they're supposed to be interesting captives). That's a list to start with anyway.
I personally keep
Bci (Colombian redtails), Dumerils, Solomon Island ground boas, and a Colombian rainbow boa. I also recently acquired a pair of Cuban boas that seem quite nice so far, but I've read the temperament can be 'iffy', so that still remains to be seen.
Raven