Question: For the University of Southampton in 2015, on which criteria did it score the best?
SQL: SELECT T1.criteria_name FROM ranking_criteria AS T1 INNER JOIN university_ranking_year AS T2 ON T1.id = T2.ranking_criteria_id INNER JOIN university AS T3 ON T3.id = T2.university_id WHERE T3.university_name = 'University of Southampton' AND T2.year = 2015 ORDER BY T2.score DESC LIMIT 1