jenkins: Sorry, I did not see your post until now. And sorry for making it sound complicated with my home-made terms. David Allen uses the word "priority" for many different kinds of things and I saw a need to distinguish some of them in order to determine how to best deal with each one.
One of the senses in which David Allen uses the word "priority" is for your "top pick right now" - whatever that choice is and for whatever reason it was that you chose it. For example, you may have lots of urgent and important work to do in the office, but you happen to be standing next to a hardware store and decide to knock off a task titled "Buy hammer" first of all (probably because you are in the right context). It need not necessarily be important or urgent or anything, but it is what you voluntarily chose to do, all aspects considered. It is in fact your top pick. That is the one I named "momentary priority" (probably not the best term for it). David just calls it "priority".
Then David also mentions the fact that some things simply are more important and urgent than others, generally speaking. He says it is usually quite easy to tell which things are more important and urgent overall. No rocket science required, and I agree. These priorities are quite stable as they are derived from your upper horizons (goals, visions etc). David calls this just "priority", too. This kind of more stable priority is one of the four factors that determine what your top pick is going to be. Your top pick, the "momentary priority" (fickle priority), is decided by your gut by combining the aspects of context, energy, time and (stable) priority. This stable priority is the one I referred to as "fourth factor priority" (probably not the best possible term either). In the example above maybe you have stuff in the office that has a high "fourth factor" priority (important, urgent etc - you really MUST do it SOON), and maybe the hammer purchase has not (perhaps no hurry at all, really), but you still decide (for some reason) to buy the hammer first, since you have a good opportunity now (and maybe it is on sale, too).
As for my own system, I do not really think about "momentary priority". I just choose and do. It is intuitive. I have no coding for it, no dates, no alarms, no nothing. But I do have coding for area/project, for context and for something similar to "fourth factor" priority. So I can look at these three (and sort and group and filter and whatever) and then choose which tasks to pick right now.
As for the "fourth factor" priority I do not actually try to assess aspects like importance, urgency etc. I take a more operational approach. I ask myself how often
at least I want to review this task (and consider whether now is a good time to do it). If the answer is "multiple times per day" I mark it red (high). If the answer is "at least once per day (in my morning scan)" I mark it blue (medium), and if the answer is that "once a week is enough" I mark it turquoise (low). (In addition, I see the tasks whenever I look for a particular context or focus on a particular area/project etc, but the above times are the guaranteed minimum).
I have found that this "review attention" coding is extremely stable, and the color helps me spot what I need very easily. It saves me time not to have to look at everything every morning. It saves me unnecessary anxiety to know that I can easily find my critical tasks even if I have not starred them tentatively for action today (just look for any red tasks; not many of those and the red bar does stand out). It allows me to keep lots of not-so-urgent stuff on my next list - I have no need for deprioritizing these into Someday and thereby miss out on good opportunities to get them done.
Ah, that last sentence made me remember. David Allen also talks about shoving stuff into Someday to be able to focus more clearly on what you have decided to do first. For some reason he does not use the term prioritization for this, but that is just what it is. It is one of the core GTD tricks, but I personally do not use it (I do not like the downside of it).