How to make Ubuntu the default OS?
According to this link,
help.ubuntu.com/community/Grub2/Setup#Configuring_GRUB_2
GRUB_DEFAULT=0 should work, if you are arriving at Ubuntu's grub menu. But there is a second step: You must make it active by running
sudo update-grub
If it is still not working, booting is not using this grub menu, but some other grub menu or other boot menu.
You may have to run
sudo grub-install /dev/sdx
where x is the drive that the computer is booting from. See this link,
help.ubuntu.com/community/Grub2/Installing#Reinstalling_GRUB_2