I too cannot choose any of the virtues of knighthood above the other.
However, if I had to choose one that could stand alone it would be prowess.
No matter how fine a man's words be, no matter how fine a man's intentions be, if he does not posses the skill to fulfill those aspirations and goals, what has he achieved but blowing air into the wind?
A knight must be ready to lead by personal example. One who lacks prowess in what they feel is their true knightly calling will simply make a fool of themselves, similar to a knight who in the name of bravery, charges their horse headlong into a formation of pikes.
Knights are doers. They always have been, and always shall be.
One must not misinterpret my massage here though. I am not advocating that what makes a man a knight is solely skill. A true knight needs all of the other virtues to be a knight in the truest sense.
However, the ability to affect the world, make things happen, and make beautiful dreams reality is what separates a knight from a well-intentioned man. While I am not decrying the well-intentioned man in any sense (They are a sort that is sorely missed and of which more is needed), he is separated from a knight by the ability to accomplish great things. They both may be quite noble, but 'tis the knight who quests and fights, and bleeds for his cause.