Perspective, the size relationship of near/far objects, is a function of camera-to-subject distance not focal length. Thus the question you should be asking is, "What is the ideal distance for portraits?"

Try this simple test. Put your nose against a mirror. Note how much bigger your nose looks relative to your ears. Now walk backwards until you think you look "normal". That will probably be at a distance of 3-1/2 to 4 feet. Now double that (to take the reflected path into account) and that's your ideal portrait distance.

Still not convinced? Try this test. Take a 50mm lens (or anything in that range) and shoot frames of a person from 4,5,6,7,8,9,10,11, and 12 feet (at the same focal length if using zoom lens). Then in Photoshop crop so all the heads are the same size. Display them side-by-side and compare how the appearance of the face changes as the distance increases. The same test can be done with a 24mm or 35mm with the same results. Perspective even looks "normal" with a 10mm if the camera is level and about 8ft from the face.

Shooting from less than about 8ft is usually less flattering than from 8ft or further away. If a face is extremely thin and deep or thin and angular it may be necessary to shoot from 15-20ft away to capture the most flattering perspective. Shooting from way far away with a long lens is a commonly used fashion / glamor technique, especially for full body shots where the legs are forward of the torso.

So there is no "one size fits all" perfect portrait lens any more than all faces are the same.The only way you find out what flatters a face the most is studying it from all angles - full-face, oblique x 2, and profile x 2 - and various distances. Find the angle and distance that flatters the person the most, then and only then pick the focal length which will produce the desired in-camera cropping.

If we don't have the ideal lens for that most flattering distance it is better to sacrifice a few pixels to shoot from further away and crop in post than to capture the image from closer less flattering distance. We can improve resolution with Unsharp Masking (USM) in post processing, but we can't change perspective.