The easiest answer is: whichever one you prefer after testing.
Assuming you have a full-frame 645 system (e.g. IQ4 150mp), I'd recommend working with your Phase One dealer to test the following:
- 110LS
- 150/2.8LS
- 150/3.5LS
- 240LS
Personally I prefer the 110 if deep in the field (e.g. 12-hour day shooting a wedding) and 150/2.8 if in the studio because the latter is pretty magical, but is a a pretty chunky lens to be carrying for a long day. We have all of these lenses available to test at no cost in our NYC or LA offices, or available to send to you as a rental that counts toward purchase. We also have a very extensive raw file catalog built up from years of our in-house testing.
Obviously 240 is quite a bit longer than the traditional definition of a portrait lens, but in a blue-sky sense (starting your search broad) you should test it before you dismiss it for this use.
If you're using a crop-sensor like the IQ3 50mp I would add the 80LS and remove the 240LS from that list.
A good Phase One dealer can arrange for you to test any/all of those lenses, and can send you portrait raw files. If you don't have a dealer, and are located in the USA, we'd welcome a chance to earn your business.