Not to mention, Bonner's the only one of the three who, IIRC, we get any sort of insight into. We actually get to see something resembling his thoughts at a couple points.For me, Arika is the best by far...
My reasons:
1) Her strong personality makes her fall a great process for any ntr fan.
2) She is also young, 20 yo I think, Lord Bonner maybe 40-50yo. The difference between ages adds a spice to the mix.
3) Her being the one with the closest relation with Kurt and also being him young and shy with sex, makes Bonner ( an experienced old man ) with much more stamina and similar strong personality a good villain.
4) Bonner being genuinely in love with Arika adds a good plot, because he feels something real for her and wants her to be pregnant with his seed.
5) The Way of a Sex Knight, ( as some of us called her final corruption process ). Her desire to serve Bonner with loyalty even knowing Kurt is still her partner (also her first and only love) and being proud of that is just peak NTR.
This of course is just my opinion, but for any NTR connoisseur, the game is the peak of Takeda's work and a good example of dedication in story plot, drawing and animation for other artists out there.
Basically, he's the only one who has the potenial for an arc/development...which will be realized if he actually comes to love Arika.