Wordpress - What is the very earliest action hook you can call?

muplugins_loaded is the earliest hook.

Depending on your wordpress setup, you may not have any plugins in the MU_PLUGINS directory. In that case this hook may not fire.

The next best hook to trigger is plugins_loaded.

RESOURCES

1. WordPress Codex - Action Reference List
2. Q/A: How to get WordPress' hook run sequence?
3. Q/A: Make sense of WP Load Core

For anyone interested in finding this out on your own, there is an awesome plugin named Query Monitor.

Install and activate the plugin (on staging or dev install obviously) and just select what you want from the drop down.