Drupal - What is the easiest way to install clean Drupal from scratch using drush?

drush qd

Downloads Drupal, runs site install, launches a lightweight webserver, opens a browser and logs you in as 'admin'.

I might have answered drush quick-drupal, but Clive said there might be a contest. :)

You may try the following few-line example:

$ drush -y dl && cd drupal-7* # Download Drupal and enter folder.
$ drush -y si --db-url="sqlite://db.sqlite" --account-pass=admin # Install.
$ drush rs # Run HTTP server.

Your site should be accessible under http://127.0.0.1:8888.

Related: Provide a one command to install and run Drupal command.