Short answer: All three of them.
Long answer: The two with the black coated blades are presents from friends and even they see lots of pocket time, I don't want to scratch the blades and I'm not using them a lot...
The Otanashi Noh Ken appears big, but it has deep carry clip and it's actually very flat knife, so it disappears in my pocket and it's very comfortable to carry,
the Hissatsu is very comfortable knife IMO, I love the AO and the auto locking gimmick. This would be the knife I would carry the most if I could find a used-one, so I wouldn't ruin the one that was a gift.
From all three knifes the most use sees the Heiho, because I'm not worrying to damage any coating on the blade.
This is the thing with those three - Mr Williams did something very smart IMO when he develop these knives, in all three of them he preserved almost identically the measurements where your index finger falls on the frame and where your thumb will be,
this way all three of them feels exactly in the same in your hand, never mind the longer or shorter blade and handle... If you used to use one of them, switching to longer or shorter blade is not an issue of any kind ( besides the fact that the Otanashi isn't AO)
So in my particular case all of them are my favorites but I'm using the most the smallest-one. If I had a satin blade Hissatsu, I would carry it the most. ONK is for cases where you cary folder strictly as a weapon option, because of the size and form of the handle, it is not very comfortable for every day utility tasks... Not like you can't use it, it's just not very comfortable IMO.
Sorry for the long answer, hope it'll help.