In which year did the user who gave the most number of "5" star reviews join the Yelp?
year the user join the Yelp refers to user_yelping_since_year; star reviews refers to review_stars;
SELECT T2.user_yelping_since_year FROM Reviews AS T1 INNER JOIN Users AS T2 ON T1.user_id = T2.user_id WHERE T1.review_stars = 5 GROUP BY T2.user_yelping_since_year ORDER BY COUNT(T1.review_stars) DESC LIMIT 1