Update a User Look up field to the User record running the workflow

I have a requirement to create an on demand workflow which will set the user record  that runs a workflow to a user look up field on the entity.

So on the entity Account there is a new field called Approved By,

A simple Workflow called "Approve Account" will just update the Approved By field to the user that ran the work flow.

But am not sure of the Dynamic Value to Pick, Accounts "Modified By" looks like a smart idea but am not sure 

The workflow is also real time and it is also  executed as the user who made changes to the record.

Will appreciate helpful suggestions.