Hello,
The answer can differ depending on the type of login you are looking for. The most common usernames are the cPanel and the Account Management Panel (AMP). After that,t he most common usernames are for the two most popularly used site builders, WordPress and the Premium Website Builder. Below is an article that goes over how all of these for you or your client.
What is my username and password (I forgot)?
Kindest Regards,
Scott M