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 a Drupal administrator theme then this is the right time to switch.
Here are some of my recent favorite Drupal 8 and 9 theme options for next-level admin experience.
1. Gin Admin Theme
Gin Admin Theme will take you into the future of how Drupal admin will look. Gin admin theme is developed by one of the lead designers of the Claro Theme. And with this, you can experience everything which is currently out of the scope of the Claro theme and future experimental customization.
Two things that I personally like in the Gin admin theme are its support for dark mode and clean admin navigation system. And it is compatible with Drupal 8, 9 and 10.
Project page: https://www.drupal.org/project/gin
2. Adminimal - Responsive Administration Theme
Adminimal is my favorite and the most popular contributed administrator theme in terms of downloads for the Drupal backend. Adminimal admin theme uses seven themes as a base theme and makes it less prone to bugs.
It is highly recommended to use the “Adminimal Admin Toolbar” which provides a minimalist style to Admin Toolbar in Drupal admin.
Project Page: https://www.drupal.org/project/adminimal_theme
3. Mediteran administrator
"Mediteran administrator" is a bluish, clean, and modern admin for Drupal 8 and 9. It is a responsive Drupal admin theme that is optimized for mobile, tablet, and desktop.
You can also combine the "Mediteran administrator" theme with "Admin Toolbar Mediteran" to get a clean look at the Admin Toolbar as well.
Project Page: https://www.drupal.org/project/mediteran
4. Claro
Claro is an all-new Drupal 8 and 9 admin theme added from the 8.8.0 release. Claro theme brings a modern admin design to drupal out of the box.
Claro admin theme is built in compliance with Admin UI & JavaScript Modernisation initiative to produce a re-imagined content authoring and site administration experience.
5. Material Admin
"Material Admin" is based on Google Material Design and modernizes Drupal 8 and 9 administration UX and design.
This Drupal admin theme takes the mobile-first approach and is great for people who want to access the Drupal 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.