How Can I Set The Default Login As User?

When using the quick "Login As" feature, iControlWP will try to log you in as the "first" admin user it finds, or it will use the user you set as default.

You can change your default login as user for a given site from the User management tab for that site.

Read here on how to login as any user.

To set the default user, simply select the user you want and check the checkbox beside the "Login As" button.