How can I prioritize Jira items from 1 to "n"? If I have 20 items assigned to a developer, I need to prioritize those items from 1 (highest) to 20 (lowest), without allowing duplicate priorities. And I need an efficient way of re-assigning priorities.

[Dan Hoffman]

If you create an Agile board, you can use the rank to prioritize.  Enable swim lanes on the board by assignee, and then drag & drop the tickets under each developer so they appear in priority order.  Jira updates the ranks of the tickets as you preposition items in the list.

[David Thom]

The prioritization feature that's presently in Jira isn't granular enough.