For importing an STL the commercial Carbide 3D options are:
MeshCAM:
or Alibre Atom3D Workshop if you also need CAM:
and Carbide Create Pro:
For the specifics on the latter, see:
A free program to replace PathCAM is Carbide Create v6:
There are various free and opensource options, and other commercial ones. What sort of STLs are you working with and how do you intend to approach them?