org-default-priority

org-default-priority is a variable defined in `org.el'.
Its value is

66

Documentation:
The default priority of TODO items.
This is the priority an item gets if no explicit priority is given.
When starting to cycle on an empty priority the first step in the cycle
depends on `org-priority-start-cycle-with-default'. The resulting first
step priority must not exceed the range from `org-highest-priority' to
`org-lowest-priority' which means that `org-default-priority' has to be
in this range exclusive or inclusive the range boundaries. Else the
first step refuses to set the default and the second will fall back
to (depending on the command used) the highest or lowest priority.

You can customize this variable.