Get unique ids based on order by another column using group by with row_number
Try this:

select reqID FROM(
select req.reqId, req.RN from (
select 
        (ROW_NUMBER() OVER (ORDER BY A.received desc)) as RN,
        A.reqId as reqId                            
        FROM RequisitionToPatient B
        INNER JOIN Requisition A ON A.reqId = B.reqId
        INNER JOIN Orders C on C.reqId = A.reqId
) req
group by req.reqId, req.RN
order by req.RN) AS a
  GROUP BY reqID


Output:

reqID
------
12
14
11
10
9