What sort of Portrait (face, shoulders & head, waist up, full length) ?
Depends on available space (as I have found out

).
I use a 50mm prime f1.4 for half body shots, a 85mm f1.8 prime for head shots. I also use the 28mm - 135mm IS which I find surprisingly good for this. It is also a great walkabout lens. Examples can be found in my 'sticky' thread on page 2.
Sport usually requires a long fast lens so look for large focal length with a small fx.x number. Others will be able to advise better on this as it is not an area I have much interest in.
Regards
RB