Which category has the highest number of users?
highest number of users refers to MAX(COUNT(app_id));
SELECT T.category FROM ( SELECT T2.category, COUNT(T1.app_id) AS num FROM app_labels AS T1 INNER JOIN label_categories AS T2 ON T2.label_id = T1.label_id GROUP BY T1.app_id, T2.category ) AS T ORDER BY T.num DESC LIMIT 1