The STM's 2017 annual report is out and on their website: http://www.stm.info/fr/a-propos/informations-entreprise-et-financieres/budgets-et-rapports-dentreprise/rapports?utm_campaign=carrousel&utm_source=ra2017&utm_medium=carrousel
I'm really surprised to find that the 121 has the highest ridership in the system, followed by 139, 141, 51 and 165.