On FF, the 'classic' portrait focal lengths are 35mm for 'environmental' portraits (where you want to include the subject's environment in the shot), 50mm for full body or small groups, 85mm for torso shots, and 135mm for head shots. Certainly, if you have more room to work with, you can use longer focal lengths. Subject features generally benefit somewhat from telephoto compression, which is why short tele lenses are better for tight portraits.


Partly, it comes down to the type of shots you like - on my 7D, I really liked shooting with the 85mm f/1.8 - so much that I moved up to the 85mm f/1.2L II. After getting 5DII, I subsequently also got the 135mm f/2L for the equivalent focal length (and aperture, since the crop factor also affects aperture in terms of DoF).


If you prefer looser framing, are shooting small groups, etc., 35mm would be a great focal length on APS-C. Dan nicely summarized the choices in that range.