There are lots of options out there. Which one is best for you will be determined by your budget, what type of portraits you want to take (tight head shots, waist up poses, environmental, etc.) and what kind of look you are going for.
I have an Anniversary Speed Graphic that I recently restored and have used several different lenses for portraiture with it. I tend to like my Tele-Xenar 360mm f/5.5 for head shots. I also use a B&L Tessar marked as a 5x7, (but is about 8 1/2" f/4.5 lens) which works well for head and shoulder shots so long as the background isn't too busy (I'm not a huge fan of the Tessar bokeh). My 150mm f/5.6 Fujinon NW also works pretty well for head and torso shots (or just about anything really). I also have a 135mm f/4.5 CZJ Tessar that's calibrated to my rangefinder that I use for candid, handheld portraits and portraits taken with my Heiland flash gun (giving them that Weegee look). For outdoor portraits in nature, I enjoy my 8 1/4" f/5.6 Petzval.
One thing I've learned with portraits is you can't shoot everyone the same way. Some people need a sharp lens, with a narrow angle, tightly cropped on their face with hard lighting, and some people need a soft lens, with a wide angle, bringing in some body posture while using soft lighting. So to me, every lens is a portrait lens. I'd hate to be limited to just one lens for all of my portraits. Now if you're just wanting to shoot one style of portrait, like adult female head shots in studio lighting, then I could see the purpose in buying a dedicated lens just for that.