My best knife - definitly my SS4! It gets the most use because of the size, it has the mudder handle which is my fave and is made out of the worlds finest steel.
As much as I like the DF which because of it's size is what I would call the flagship ,I prefer the SS4. As a fighting tool the DF with a sharpened back edge would be my first choice. At close range I would take it even over a handgun.
I don't have a S6 but I have an M6 which is almost as good. The S6/M6 is an awesome fighter and all around user and would be a great choice as a military knife. I can't wait to compare the M6 to the S5(dumpster Mutt).