Ubuntu Change Preferred Default Boot Order

You have dual boot system; Ubuntu and Win7.
And you like to auto boot into the previous booted system.

Change GRUB_DEFAULT to "saved" and GRUB_SAVEDEFAULT to "true"

$ sudo vi /etc/default/grub
#GRUB_DEFAULT=0
GRUB_DEFAULT=saved
GRUB_SAVEDEFAULT=true
#GRUB_HIDDEN_TIMEOUT=0
GRUB_HIDDEN_TIMEOUT_QUIET=true
GRUB_TIMEOUT=10
GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
GRUB_CMDLINE_LINUX=""

$ sudo update-grub
 
Now do a reboot and give it a try