I think there is not single lens that would work equally well for all situations and any kind of portrait. Today, a friend asked my opinion on which lens would be better for a head-to waist portrait in terms of the background blur: a 200 mm f/2.8 or an 85 mm f/1.4. This is an interesting question, as one of the lenses has wider maximum aperture and the other one has longer focal length. Both of these parameters affect the depth of field the same way – increasing either the focal length or the aperture, while keeping other parameter the same, makes the depth of field shallower (thus increasing the background blur). To what degree these parameters affect the depth of field is more difficult to say without doing a rigorous cross-comparison of the lenses. So probably, there background blur would be comparable for the two lenses.
There is another factor to consider when choosing between these lenses – the distance between the camera and the subject. For a head-to-waist portrait, I would prefer a 200 mm lens, as it would make me shoot from farther away, which would result in more flattering facial features. Generally, the closer the camera to the subject, the more prominent the protruding parts of the face (mainly, the nose) are. Subconsciously, most people find smaller noses more attractive. This is a gross generalization, of course, but it is, generally, a fact. it is based on the tendency of humans to associate smaller noses (and larger eyes, for that matter) with young age. The longer lenses compress the perspective in the image, so the noses appear not as prominent as in portraits shot from a closer distance with wider lenses.
I use a Canon camera, so for me, the lens of choice for portraits is a 70-200mm f/2.8L IS II USM
