Some sources list the black widow as the most poisonous. They are quite common here in AZ. From spring til late fall I routinely have several around the outside of the house. I don't bother them because they eat crickets. There are crickets everywhere here and I don't like the little bastards. Since black widows are orb weavers, they stay put in their web so if you are paying attention you won't run into one. I get them in the house once in a while, those I get rid of quickly.