My understanding is that if you check/uncheck Contact record Field 'Email Opt out (HasOptedOutOfEmail)' field in salesforce then the record/contact gets updated in Marketing Cloud.
But for some reason it is not updated or maybe it doesn't do anything at all.
So what is best way to update All Subscriber List for Unsubscription/Subscription?
You need to add the link "Marketing Cloud Unsubscribe" to your page layout in Sales/Service Cloud in order to get the status to synchronize correctly. This would be the best option to do update the status from Sales/Service Cloud. You can find this link in "Custom Links -> ExactTarget Unsubscribe" along with the Resubscribe link when editing the contact/account page layout. Other ways include doing this directly in Email Studio or via the Marketing Cloud API.
Further information can be found in the documentation about Marketing Cloud Connect - Unsubscribes.
