When creating a Channel field, such as a radio field type, is there a way to specify that a certain option should be selected by default? Same question for checkboxes, select dropdowns, etc.
There is not - although you could add some javascript to do so (using, for example, the CP CSS & JS add-on).
This would be a good feature request for Pixel and Tonic to add to their P&T Field Pack fieldtypes.