Best practice: $this->t() versus t()
Best practice depends on where the code is placed.

OOP Code

Use $this-&gt;t().

If you extend a drupal base class like a controller or a plugin the t() function is provided as class method $this-&gt;t() out of the box and you should use it. This makes your code testable.

For most tasks you'll find a suitable drupal class to extend from which has $this-&gt;t() defined, but if you need to build your own class from scratch best practice would be to use the string translation trait and inject this as service if you use this class in a service context:

use Drupal\Core\StringTranslation\StringTranslationTrait;
use Drupal\Core\StringTranslation\TranslationInterface;
class MyClass {
  use StringTranslationTrait;

  /**
   * Constructs a MyClass object.
   *
   * @param \Drupal\Core\StringTranslation\TranslationInterface $string_translation
   *   The string translation service.
   */
  public function __construct(TranslationInterface $string_translation) {
    // You can skip injecting this service, the trait will fall back to \Drupal::translation()
    // but it is recommended to do so, for easier testability,
    $this-&gt;stringTranslation = $string_translation;
  }

  /**
   * Does something.
   */
  public function doSth() {
    // ...
    $string = $this-&gt;t('Something');
    // ...
  }

}


Source: https://www.drupal.org/docs/8/api/translation-api-code-text

Procedural Code

Use t().

If you have procedural code, for example a hook, then use t(), which is a global function.

Not best practice would be to use the procedural t() in OOP Code.