There is no one focal length lens that is "perfect" for portraits. I would say that the closest are the 70-200 2.8 zooms
from Canon or Nikon.
Different faces benefit from different focal lengths.
Also how close you are cropping on the face makes a big difference.
With the high resolution and pixel count of MF (as well as top of the line 35mm DSLR) you can get away with cropping the file in post
to get the right level of compression for the type of face you are photographing.
Here is a interesting comparison made by Stephen Eastwoodhttp://stepheneastwood.com/tutorials/lensdistortion/strippage.htm
It's with 35mm DSLR, but the same applies to medium format, just convert the focal length.
I would also add that more or less compression also effects the feel of an image or at lest supports/reinforces it.
That said a 150mm on a digital back is a good lens for portraits on most faces and if you are not shooting too close.