Depends on the camera you are using, mainly if its a crop sensor camera.
While not a portrait shooter (still learning), I've read that a great focal lenght for portraits is between 85mm and 105mm, on a full frame.
So if you are shooting with a crop sensor camera, that would be between 50mm and 70mm.
I'd say a 50mm 1.4 would be a great portrait lens, or even maybe a 24-70?