The best way to install Drupal 8 and 9 is by using Composer. There are several options, but the most widely used and easiest way is to use the Composer template for Drupal projects. To install Drupal 8 or 9 use the following command:
composer create-project drupal-composer/drupal-project:8.x-dev some-dir --no-interaction
or if you want to install it in the current directory use this:
composer create-project drupal-composer/drupal-project:8.x-dev . --no-interaction
To update the existing project that is based on this template use the following command:
composer update drupal/core webflo/drupal-core-require-dev symfony/* --with-dependencies
Update on October 24, 2020.
As of the release of Drupal version 8.8, the recommend Composer template changed from drupal-composer/drupal-project to the officially supported template drupal/recommended-project.
To create a new project in the 'my_site' directory and automatically execute composer install, use the following command:
composer create-project drupal/recommended-project my_site
The above command will download the current release of Drupal. If you want to download a different version, add the version number to the command. For example, to download version 8.9.7:
composer create-project drupal/recommended-project:8.9.7 my_site