A query to list authors / users by post count:
SELECT wp_users.ID, wp_users.user_nicename, COUNT(*) as count FROM wp_posts, wp_users WHERE wp_posts.post_type='post' AND wp_posts.post_status='publish' AND wp_posts.post_author = wp_users.ID GROUP BY post_author ORDER BY count DESC LIMIT 5 ;
Resulting table:
ID,user_nicename,count 29,"user-a",18 66,"user-b",16 26,"user-c",10 24,"user-f",9 48,"user-z",6