#Sqlserver 2008, 250 bytes

DECLARE @ varchar(max) = 'That''s... that''s... that is just terrible!" he said.';
WITH c as(SELECT
@ p,@ x
UNION ALL
SELECT LEFT(x,k-1),STUFF(x,1,k,'')FROM
c CROSS APPLY(SELECT patindex('%[^a-z''-]%',x+'!')k)k
WHERE''<x)SELECT max(p)FROM(SELECT top 1with ties p
FROM c WHERE p>''GROUP BY p
ORDER BY count(*)DESC
)j HAVING count(*)=1

Try it online!

#Sqlserver 2016, 174 bytes

Unable to handle data like this example(counting the equals as 3 words):

DECLARE @ varchar(max) = 'That''s... that''s... that is just terrible!" he said. = = ='
SELECT max(v)FROM(SELECT TOP 1WITH TIES value v
FROM STRING_SPLIT(REPLACE(REPLACE(REPLACE(@,'"',''),',',''),'.',''),' ')GROUP
BY value ORDER BY count(*)DESC)x HAVING count(*)=1

Sqlserver 2008, 250 bytes

DECLARE @ varchar(max) = 'That''s... that''s... that is just terrible!" he said.';
WITH c as(SELECT
@ p,@ x
UNION ALL
SELECT LEFT(x,k-1),STUFF(x,1,k,'')FROM
c CROSS APPLY(SELECT patindex('%[^a-z''-]%',x+'!')k)k
WHERE''<x)SELECT max(p)FROM(SELECT top 1with ties p
FROM c WHERE p>''GROUP BY p
ORDER BY count(*)DESC
)j HAVING count(*)=1

Try it online!

Sqlserver 2016, 174 bytes

Unable to handle data like this example(counting the equals as 3 words):

DECLARE @ varchar(max) = 'That''s... that''s... that is just terrible!" he said. = = ='
SELECT max(v)FROM(SELECT TOP 1WITH TIES value v
FROM STRING_SPLIT(REPLACE(REPLACE(REPLACE(@,'"',''),',',''),'.',''),' ')GROUP
BY value ORDER BY count(*)DESC)x HAVING count(*)=1

#Sqlserver 2008, 250 bytes

DECLARE @ varchar(max) = 'That''s... that''s... that is just terrible!" he said.';
WITH c as(SELECT
@ p,@ x
UNION ALL
SELECT LEFT(x,k-1),STUFF(x,1,k,'')FROM
c CROSS APPLY(SELECT patindex('%[^a-z''-]%',x+'!')k)k
WHERE''<x)SELECT max(p)FROM(SELECT top 1with ties p
FROM c WHERE p>''GROUP BY p
ORDER BY count(*)DESC
)j HAVING count(*)=1

Try it online!

#Sqlserver 2008, 250 bytes

DECLARE @ varchar(max) = 'That''s... that''s... that is just terrible!" he said.';
WITH c as(SELECT
@ p,@ x
UNION ALL
SELECT LEFT(x,k-1),STUFF(x,1,k,'')FROM
c CROSS APPLY(SELECT patindex('%[^a-z''-]%',x+'!')k)k
WHERE''<x)SELECT max(p)FROM(SELECT top 1with ties p
FROM c WHERE p>''GROUP BY p
ORDER BY count(*)DESC
)j HAVING count(*)=1

Try it online!

#Sqlserver 2016, 174 bytes

Unable to handle data like this example(counting the equals as 3 words):

DECLARE @ varchar(max) = 'That''s... that''s... that is just terrible!" he said. = = ='
SELECT max(v)FROM(SELECT TOP 1WITH TIES value v
FROM STRING_SPLIT(REPLACE(REPLACE(REPLACE(@,'"',''),',',''),'.',''),' ')GROUP
BY value ORDER BY count(*)DESC)x HAVING count(*)=1

#Sqlserver 2008, 250 bytes

DECLARE @ varchar(max) = 'That''s... that''s... that is just terrible!" he said.';
WITH c as(SELECT
@ p,@ x
UNION ALL
SELECT LEFT(x,k-1),STUFF(x,1,k,'')FROM
c CROSS APPLY(SELECT patindex('%[^a-z''-]%',x+'!')k)k
WHERE''<x)SELECT max(p)FROM(SELECT top 1with ties p
FROM c WHERE p>''GROUP BY p
ORDER BY count(*)DESC
)j HAVING count(*)=1

Test Here

#Sqlserver 2008, 250 bytes

DECLARE @ varchar(max) = 'That''s... that''s... that is just terrible!" he said.';
WITH c as(SELECT
@ p,@ x
UNION ALL
SELECT LEFT(x,k-1),STUFF(x,1,k,'')FROM
c CROSS APPLY(SELECT patindex('%[^a-z''-]%',x+'!')k)k
WHERE''<x)SELECT max(p)FROM(SELECT top 1with ties p
FROM c WHERE p>''GROUP BY p
ORDER BY count(*)DESC
)j HAVING count(*)=1

Try it online!