When you run visudo or crontab -e on a Ubuntu/Debian server, you will be using default editor.

To set default editor, run

update-alternatives --config editor

To see all available editors, run

update-alternatives --query editor

Example

root@backup:~# update-alternatives --query editor
Name: editor
Link: /usr/bin/editor
Slaves:
 editor.1.gz /usr/share/man/man1/editor.1.gz
 editor.fr.1.gz /usr/share/man/fr/man1/editor.1.gz
 editor.it.1.gz /usr/share/man/it/man1/editor.1.gz
 editor.ja.1.gz /usr/share/man/ja/man1/editor.1.gz
 editor.pl.1.gz /usr/share/man/pl/man1/editor.1.gz
 editor.ru.1.gz /usr/share/man/ru/man1/editor.1.gz
Status: manual
Best: /bin/nano
Value: /usr/bin/vim.basic

Alternative: /bin/ed
Priority: -100
Slaves:
 editor.1.gz /usr/share/man/man1/ed.1.gz

Alternative: /bin/nano
Priority: 40
Slaves:
 editor.1.gz /usr/share/man/man1/nano.1.gz

Alternative: /usr/bin/vim.basic
Priority: 30
Slaves:
 editor.1.gz /usr/share/man/man1/vim.1.gz
 editor.fr.1.gz /usr/share/man/fr/man1/vim.1.gz
 editor.it.1.gz /usr/share/man/it/man1/vim.1.gz
 editor.ja.1.gz /usr/share/man/ja/man1/vim.1.gz
 editor.pl.1.gz /usr/share/man/pl/man1/vim.1.gz
 editor.ru.1.gz /usr/share/man/ru/man1/vim.1.gz

Alternative: /usr/bin/vim.tiny
Priority: 10
Slaves:
 editor.1.gz /usr/share/man/man1/vim.1.gz
 editor.fr.1.gz /usr/share/man/fr/man1/vim.1.gz
 editor.it.1.gz /usr/share/man/it/man1/vim.1.gz
 editor.ja.1.gz /usr/share/man/ja/man1/vim.1.gz
 editor.pl.1.gz /usr/share/man/pl/man1/vim.1.gz
 editor.ru.1.gz /usr/share/man/ru/man1/vim.1.gz
root@backup:~#