Regarding the best FL for portrait work, it rather depends on what you call a "portrait", but for tightly cropped head-and-shoulders shots, the most realistic perspective was (in the days of 35mm film) reckoned to be 75-80mm. Much shorter than this would accentuate facial features, and longer would flatten them.
This link has been posted before, but illustrates beautifully what happens for different FLs:
Stephen Eastwood|Beauty and Fashion Photographer | Tutorials
Note that these examples were for 35mm film, so for APS-C, you would need to divide by 1.5.
How close you want to get to your subject will have a bearing on the chosen FL, but for the best perspective, you must resign yourself to getting quite close (4-5 feet, say).
Alternatively, you may want a more loosely cropped shot, in which case you will perhaps be better off with shorter FLs than "optimum". An example of this would be G-Diesel's shot (above), shot at 16mm (which certainly isn't a normally prescribed FL for "portraits"!).