MySQL unisci due tabelle, trova il conteggio massimo e ordina per

select daybook.userid, count(*) as count 
    from daybook, rating 
    where daybook.userid = rating.userid
    group by daybook.userid
    order by count desc

Ma non hai nemmeno davvero bisogno della tabella dell'agenda:

select userid, count(*) as count
   from rating
   group by userid
   order by count desc