Debian uses for visudo, crontab, reportbug and other similar applications the default text editor, if you want to change it use the update-alternatives command.
Run this on a CLI.
root# update-alternatives --config editor
The options in my Debian system are:
There are 6 alternatives which provide `editor’.
Selection Alternative
———————————————–
1 /bin/ed
2 /bin/nano
3 /usr/bin/vim.tiny
*+ 4 /usr/bin/vim.gnome
5 /usr/bin/mcedit-debian
6 /usr/bin/emacs21
Press enter to keep the default[*], or type selection number:
The + sign denotes the default option, and the * sign denotes the actual selection, to change from vim to nano, just press 2 in this example and the ENTER.