The earliest admin related hook I can see is admin_menu, so before that comes (listing only useful core hooks, from later to earlier)
- auth_redirect
- update_option
- wp_loaded
- init
- pre_get_posts
- after_setup_theme
- get_template_part
- setup_theme
- plugins_loaded
- muplugins_loaded
What are you trying to do, exactly? the complete answer depends on your use case.