You should be able to use the MAX to get what you want, unless I'm not understanding your issue.

DECLARE @sample TABLE
(
    ContactID INT,
    RenewalDate DATE
)
;

INSERT INTO @sample
SELECT 1, '01/07/2018'
UNION ALL
SELECT 1, '01/07/2017'
UNION ALL
SELECT 1, '01/07/2016'
UNION ALL
SELECT 2, '01/07/2015'
UNION ALL
SELECT 2, '01/07/2014'
UNION ALL
SELECT 3, '01/07/2018'
UNION ALL
SELECT 3, '01/07/2017'
UNION ALL
SELECT 3, '01/07/2016'

SELECT ContactID, MAX(RenewalDate) AS RenewalDate
FROM @sample
GROUP BY ContactID
ORDER BY ContactID
Results:
ContactID    RenewalDate
     1        2018-01-07
     2        2015-01-07
     3        2018-01-07