Drupal administrator interface was never considered great in terms of catching up with modern design and standards. And if you are one of those people who always use "Seven" as Drupal administrator theme then this is the right time to switch.
Here are some of my recent favorite theme options for the Drupal 8 administrator interface for the next level admin experience.
1. Adminimal - Responsive Administration Theme
Adminimal is my favorite and the most popular contributed administrator theme for Drupal backend. Adminimal admin theme uses seven themes as a base theme and makes it less prone to bugs.
Project Page: https://www.drupal.org/project/adminimal_theme
2. Mediteran administrator
"Mediteran administrator" is a bluish, clean and modern admin for Drupal 8. It is a responsive Drupal 8 admin theme that is optimized for mobile, tablet and desktop.
You can also combine the "Mediteran administrator" theme with "Admin Toolbar Mediteran" to get the clean look on the Admin Toolbar as well.
Project Page: https://www.drupal.org/project/mediteran
Claro is an all-new Drupal 8 admin theme added from 8.8.0 release. Claro theme brings a modern admin design to drupal out of the box.
4. Material Admin
"Material Admin" is based on Google Material Design and modernize Drupal 8 administration UX and design.
This Drupal 8 admin theme takes the mobile-first approach and great for people who want to use Drupal 8 backend from mobile.
Project page: https://www.drupal.org/project/material_admin
Let me know in the comment which one is your favorite or you are still not comfortable switching away from the "Seven" theme.