Mt Spokane Photography said:
300mm is also a excellent focal length. The issue is space enough to use a 300mm lens.
Longer focal lengths tend to shorten noses and facial feature which tends to be more flattering. Wide angles are the opposite. Any of the 85-300 focal lengths are fine. Its more of a pick the one that does what you need. If you want full body, for example, I'd tend to go 85, but have used 135 as well.
+1 Exactly--there is no "one size fits all." The 70-200mm is so great because it allows changing focal length for distance and the shape of a person's head. For slim subjects, 85mm works straight on, but if, as is the case with many in the West, the subject is overweight, going to a longer focal length is very important. Sometimes, even, a person who appears fit has a head that just doesn't work at certain focal lengths, so it's great to be able to show them images taken at various FL's.
Going the other way, there are some subjects that have narrow heads and long faces, and having an 85mm or even 70mm is better than going with a 135mm, for example.
But if I had to choose two primes for portraits, I'd go with 85 and 135. But if I could only have ONE lens, I'd even go with the 70-200mm f/4. Despite giving up the shallower DoF look of faster lenses, it is sharp and produces great colors, and I'd say for most portrait work where background can be controlled and one wants most of the face and body in sharp focus, it works. But the 70-200mm gives even more options with backgrounds and DoF, so, heavy as it is, I'd say it is THE portrait lens if budget allows.
Update: After sleeping on it overnight...If I had to choose one PRIME lens strictly for portraits, I'd go with the 135mm f/2. Quick, great IQ including bokeh, and does flatter most head shapes, imho. But it would not be nice iN a tight space!!!