The "classic" range for portrait lenses on 35mm full frame camera was from around 80mm to 135mm. So the zooms already mentioned, or the 24-105 (although it is a slower lens at f4) are all suitable.
A couple of articles
http://www.digital-slr-guide.com/bes...rait-lens.html
http://www.bobatkins.com/photography...it_lenses.html