It just kills me when I type git commit or some other such command on a newly configured machine and up comes nano or (worse!) pico. And, I so infrequently have to do this that I can never remember it - so, for posterity:
To set your default editor (to Vim) on Ubuntu:
update-alternatives --config editor
Ex:
$ sudo update-alternatives --config editor
There are 4 choices for the alternative editor (providing /usr/bin/editor).
Selection Path Priority Status
-----------------------------------------------------------
0 /bin/nano 40 auto mode
1 /bin/ed -100 manual mode
2 /bin/nano 40 manual mode
* 3 /usr/bin/vim.basic 30 manual mode
4 /usr/bin/vim.tiny 10 manual mode
Press <enter> to keep the current choice[*], or type selection number: 3