Hello how to can we rank this employees
First you need to define what makes your couriers the best. For example, if you only look at the deliverd orders you can simply add ORDER BY [Delivered orders] DESC to your query.
But that will be a litle bit to simple as maybe the delivery time is also a good indication. So based on your definition of the best courier you can proceed further.
I would calculate a score on multiple columns and add a percentage to it to weight the column.