Debian and Ubuntu uses for visudo, crontab and other similar applications the default text editor, you want to change it use the update-alternatives command.

It is as easy as, run this on a terminal window.

sudo 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.

Now all applications that uses an editor will use the new default one.