As I understand it, shortlists are not part of the GTD-method because for many people, things change too quickly to rely on a shortlist. The concept is to use your intuition and knowledge of what you have on your plate to make in-the-moment decisions. For many, the shortlist gets tossed out the window by coffee and everything on it gets rewritten to the next day.But as you describe, I ask myself that question ("what on this list would I be most happy about having completed") every time I've finished a task and then check the complete list.
You should ask yourself this question only 1 or 2 times a day and then isolate the tasks.
Working with a shortlist is what I want to do.
But I wonder why this list is not a part of the GTD-method?
Try using a shortlist and see how it works for you. My advice would be to not take the items off your NA list though. That way, if a phone call at 9:30 changes your entire day, you can toss that shortlist without worrying that you are losing something.