I would have to say in order from least to most nippy of the ones I have kept, they would go:

Saharans (E. muelleri)
Tatars (E. tataricus)
Desert (E. miliaris)
Spotted (E. jaculus)
Indian (E. johnii) - strong feeding response can result in accidents though!
Kenyan/East Africans (E. colubrinus)
Rough-scaleds (E. conicus)

However, almost all the Saharans available are wild caught and some of those snakes, even though docile, can be tricky to get feeding.

That said, I have been bitten by every species on this list (and Eryx jayakari as well).

If you get a captive baby of any of the species and work with it, they will be trustworthy adults, generally.
But if you want a sandboa that will never bite, get a Rubber Boa .
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Chris Harrison
San Antonio, Texas