This can all be achieved with a custom formula field and you can avoid Process Builder altogether.

Add all these conditions to the formula field, and anytime conditions don't match properly, you can put the value as `null` so it'll be blank for situations in which there shouldn't be a rating.

If you need this rating to be static and unchanging at the time it's set (and not always updating with changing conditions), have the same formula field in the background (i.e. take it off any of the page layouts) and use Process Builder to simply set the normal `Rating` custom field to the `Calculated Rating` custom formula field.