there was a thread here a long time ago, where people argued which range of focal length is the most suitable for portraits, and it boils down to your style of portraiture and the environment actually.
I guess for conventional style, 50mm is good for half or even full body shots, gives you a good working distance and proper perspectives. However, if u go in close on facial with the 50mm, u may get perspective effects like big noses and all, but that may be the style u wish to go for. And again, for conventional style, a range of 80-135mm is useful for headshots, esp with a wide f2.8 to give nice bokeh.
that said, have to disclaim that I'm not saying the above lenses are the only ones, or the best ones, for portraiture purposes, they're just what people usually use.