I don't think priorities of "low", "medium" and "high" are very helpful. You'll focus on the "high" priority tasks until it's pretty clear that the "low" and "medium" ones aren't getting done at all. More jobs will be classified as "high" priority just to make sure they get done, and a new "critical" category is usually added to differentiate the "high" priority jobs that need to come first. You can just keep climbing that ladder forever, if you want, until it starts looking ridiculous, with "hyper-critical" over "super-critical" and so on down in an ever-advancing "popcorn sizing" model.
If you want to actually set priorities, you need to compare pairs of tasks and say that one is more important than the other. Keep that up and you'll eventually get the top-level task that is more important than all the others. When that one is complete, you'll see the others below it and you can rank them relative to each other until you find the top priority among them. "Top priority" is an emergent property of knowing which tasks are more important than others. That's how you set priorities.
Mokalus of Borg
PS - You may never get to those that get voted down to the bottom.
PPS - But you weren't getting to them anyway, so what's the difference?