Select a value for the max
RANK will deal with multiple max values (instead of ROW_NUMBER)

SELECT
   dtimesent, Request, ResponseTime
FROM
    (
    select dtimesent, Request, ResponseTime ,
       RANK() OVER (ORDER BY ResponseTime DESC) AS rn
    FROM allrequests 
    where Request like '%Requestsa%'
    ) T
WHERE
   rn = 1


Also see How to get the MAX row please