1) Make two lists. Write everything down in the first list. The second list is empty.
2) Something came up! One item on this list cannot be done. It doesn't matter why: you know which one that is. Which one is it? Write that down in the second list and mark it off the first list.
3) Repeat until the first list is done.
Congratulations, your second list is prioritized (albeit likely in reverse order), without extraneous algebraic diversions.
Bonus points for going Warren Buffett on it: only the top 3 priorities matter, sideline everything else; consider repeating the exercise after those three are complete.