What is the most preferable default button for submitting a form (like a login page)? OK or Cancel?
The default button for a form is usually "OK".
If you accidentally hit return before all the data is filled in the form validation should prevent the action from completion and return you to the form ready to complete all the fields. You could even disable "OK" until all the required fields have been entered, but still leave it as the default button. In this case hitting return would do nothing.
If "Cancel" were the default you would potentially lose data and have to start all over again - which is not a very friendly thing to do.
Never "Cancel", it would be infuriating to fill out the whole form, press "enter" and lose everything.
If the information in the form is not too critical then there's no harm in submitting it "by mistake". If the information is critical, then there should be some kind of checkbox confirmation at the end of the form that will only be checked once the user has correctly filled out the whole form.
Always use "OK" as the default.