I have to do a portrait shoot for a friend. My camera (Pentax K-30) allows me to focus in the following modes:
I haven't yet understood how to effectively use an 11- (or Nikon 54-point) focus mode. In my opinion, one focus point is the best to control the focus area, and the best method is to use a single focus point then recompose the frame.
Which of the focus modes I listed is best for portrait shooting?
The best thing to do is to manually select the focus point closest to what you want to be the point of focus, and if necessary recompose only slightly from there. That's because turning the camera to recompose moves the plane of focus more than you might think — see this answer for a nice diagram.
Typically, with portraits, focusing on the eyes is considered "correct" — most people looking at portraits respond well to that, generally with the closest eye in sharpest focus. (Long ago, was a fashion for focusing on the cheekbones rather than the eyes, but these days general consensus is on eyes. Of course, you don't have to follow that.)
So, put the camera in focus point selection mode. Pick a focus point close to the subject's eye, put it right over the eye, lock focus, recompose slightly if needed, and click.
Your Pentax camera makes this easy, although it is not the default — the four-way arrow controller defaults to bringing up various functions like drive mode, white balance, etc., but if you press and hold OK, there will be a beep, and the camera will switch to moving the focus point, which can be very quick once you've practiced a bit. (Press and hold again to put it back to the function mode.)
Some cameras — usually mirrorless cameras which work with the full sensor for contrast-detect autofocus, or DSLRs in liveview — can do eye-detection autofocus. If this is an option and your camera can do it quickly enough, I highly recommend it. That lets you concentrate on the subject's expression and timing the shot rather than on making sure the focus point is aligned.
If you have the luxury to focus with the center point and recompose, then this is your best solution.
This is because the center point is the most sensible AF point and will nail the focus the best. Be sure tough to have enough DoF in order to catch the portrait in focus (unless of course if you want to achieve some special effects) and, also, focus between the eyes - usually is considered the best place to focus even if others have other opinions.
If you cannot focus+recompose then it is best, perhaps, to go for 11 points (depending on your background) because skin tones are somewhat soft and hence not enough color change for the 5 points to catch on them.
You asked „which of the PREVIOUS mode” and I tried to answer. However, by far, if your conditions permit (tripod, static subject, patience etc.) the best focusing method is Manual Focus with some sort of tool to help you - Live View (on camera's display or on a computer/tablet) or focusing screen if your camera model permits. It is by far the most precise method.
