Lots of the smallest species (ground snakes, ring necks, night snakes, blind snakes, black headed etc) don't make the best captives. Some are hard to keep alive. Others, even though they're keepable, are just boring captives (and this is from a guy who has ball pythons).

Smallest good pet snakes, IMO would probably be things like checkered garter snakes, kenyan sand boas, ground boas (CBB only)possibly spotted pythons (males)...
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We wouldn't have 6 and a half billion people if you had to be beautiful to get laid.
1.3 African House Snakes
3.2 reticulated pythons
1 corn snake
4.3 Florida Kings
2 speckled kings
1.2 ball pythons
1 Argentine boa
1 Texas Rat Snake
1 checkered garter snake