This was discussed in a previous thread where people went round and round because one group defined "portrait" in the conventional sense (head and shoulders) while the other group believed it meant a 3/4 view of the subject or even full length. So a choice of lens depends on how you define portrait. For the conventional portrait in 35mm the preferred focal length is 80 to 105 mm to avoid distortion of the subjects features. For MF and LF the same rule applies with the use of lenses that have an equivalent angle of view. All this is usually covered in books on portraiture.
SO ... before going any further we all need to agree on what "portrait" means.