In Ubuntu 16.04, I wanted to add a user cron file with crontab -e, which has a new behavior: the very first time, it asks you what editor you want to use (i.e. nano, vim, ...)
If you choose the wrong one, then next time it does not ask you, it remembers your choice forever, so you'll be stuck.
The nano editor is okay, but difficult to work with if you're used to something like vim.
The data actually gets saved in a hidden file of your home directory named .selected_editor.
# Generated by /usr/bin/select-editor
Mine was set to ...