Here's some info that includes toxicity:
http://homeguides.sfgate.com/p...
I don't know what the least toxic kind might be. I would think that Asclepias tuberosa would be safer, because I've heard it tends to lack the latex-like white sap that most milkweeds have. In my area, that species cannot be successfully grown, even though a number of organizations include it in lists of recommended native plants given out here. It's also less preferred by Monarchs, but some will use it as a host plant, I hear. One thing seldom mentioned on safety is that whenever you have contact with milkweed sap, such as when cutting it back, you should take precautions. Wash your hands well immediately after. If the sap ever should get into your eye, it can cause damage to the cornea, so that should be considered an emergency if it happens, and eyes should be carefully rinsed out before seeking medical help. The milkweed taste very bitter and the chances of a pet swallowing enough to be poisoned are small. I've had no problem with my cats. Deer will graze on the flowering parts of milkweed and I haven't yet heard of any of them dying from it around here.
