Depends on the shot.
Whole body/Family: wide 28mm or 35mm.
Upper half/Couple: 35mm or 50mm
Head and shoulders/Single: 50mm or 75mm
Of course, you have shoot fun headshots at 15mm if you want or full body shots at 75mm. I don't think there is one perfect focal length for portraits. Use what you have creatively and your shots will be great.
Model: PENTAX K20D
ISO: 100
Exposure: 1/180 sec
Aperture: 3.5
Focal Length: 28mm
Model: PENTAX K200D
ISO: 200
Exposure: 1/60 sec
Aperture: 8.0
Focal Length: 75mm
Model: PENTAX K20D
ISO: 200
Exposure: 1/60 sec
Aperture: f/5.6 I think (manual aperture)
Focal Length: 50mm
Model: PENTAX K20D
ISO: 100
Exposure: 1/180 sec
Aperture: f/5.6 I think (manual aperture)
Focal Length: 50mm
Model: PENTAX K200D
ISO: 200
Exposure: 1/500 sec
Aperture: 4.0
Focal Length: 75mm