Mike has asked a very pertinent question - but I can go further. What brand of camera are you using? Are you after fixed FL or a zoom? The reason I ask this is there a good and bad in all brands, and some favored over others in particular brands.

As Mike suggested the 24-105 is an all alrounder and certainly capable of taking a portrait shot. If you want to specialize and still want a zoom then maybe you could be looking at one of the many variant 24-70 F2.8 zooms. These both assume you are shooting inside - but if you want to do a portrait outside then certainly the 70-200 is the preferred focal length.

If you are looking at fixed focal length then the inside favorite is either the 50mm or 85mm. Even the nifty fifty (Canon 50 1.8) is a very capable portrait lens. Some photographers also use the 35mm but you need to be careful with distortion at close range. If you are outside then I would suggest either the 85 /100 / 135 focal lengths are the most popular - and experienced portrait photographers would sometime use the 200mm where the need arises.

All the above I refer to I am assuming a full frame body - but of course zooms have a little more flexibility over fixed focal lengths when it comes to crops.