more info required. budget, what kind of portraits do you want to shoot, should the lens also be suitable for other purposes, sport etc etc. I achieve for example positive results with the 50mm on my crop saensor camera, however living in an apartment even this focal length can get a bit cramped and I struggle for space between myself and subject, then I take a 35mm. Under these conditions for example an 85mm would be impossible. The more specific information you give, the more accurate answer you will get here on the forum.