What determines radical stability?
Here's a method to figure out stability of intermediates.
This method will help you in figuring out stability of all types of intermediate be it carbocation, carbanion or carbon free radical.
Just think of carbon atom as a person who neither happy with more electron density nor with less. It just needs the right amount of electrons. A free radical has one electron less than Carbon atom. So it needs to compensate for the loss of one electron.
The increasing order for this stability order is
Benzylic carbon > Allylic carbon > Tertiary carbon > Secondary carbon > Primary carbon.