Name the publisher which has the most titles published in 1991.
most title published refers to MAX(count(title_id); published in 1991 refers to YEAR(pubdate) = 1991
SELECT T2.pub_name FROM titles AS T1 INNER JOIN publishers AS T2 ON T1.pub_id = T2.pub_id WHERE STRFTIME('%Y', T1.pubdate) = '1991' GROUP BY T1.pub_id, T2.pub_name ORDER BY COUNT(T1.title_id) DESC LIMIT 1