An exclamation mark itself conveys the "Important" meaning already, so using it to denote low priority is bad idea in my opinion. So I would not use the first two.
I would not use the other two neither, because people will be scared by the important elements. Probably it is not your intent for the app to do that.
I would use:
- something prominent for high priority, e.g.: + or ↑
- something neutral for normal priority, e.g.: •
- something diminished for low priority, e.g.: - or ↓
I think it is important for these symbols to correspond to each other:
- the neutral one only should indicate that there is an option to change it, but should not get in the way too much
- low priority should be at the opposite side of high priority.
This way you would have a set of icons like these for example: ↑ • ↓
Of course, you can go further, making the circle gray, the up arrow bold and red, and the down arrow thin and light gray.