The most classically chosen portrait focal length is 85mm, which represents an excellent balance of all the characteristics sought after by photographers. However, portrait photographers will frequently employ a wide range of focal lengths depending on the look they are going for. This great video compares some of the best lenses at 85mm, 105mm, 135mm, and 200mm.
Coming to you from Manny Ortiz, this awesome video takes a look at the Canon RF 85mm f/1.2L USM, Nikon AF-S NIKKOR 105mm f/1.4E ED, Sony FE 135mm f/1.8, and Canon EF 200mm f/2L IS USM lenses. No doubt, these are all superlative lenses, and you can create fantastic images with any of them, but it is quite interesting to see just how differently each of them renders images, particularly as you get into the increasing levels of telephoto compression offered by the longer focal lengths. Even if you are not interested in dropping top dollar on these specific lenses, remember that the perspective you see from each focal length will apply to other lenses of the same focal length; it is just things like sharpness, bokeh rendering, maximum aperture, etc. that change. Check out the video above for the full rundown from Ortiz.