The MBTA's annual Ridership and Service Statistics report, a.k.a., "the blue book" was released on May 10.
This report includes a true treasure-trove of statistics through which to pour - most popular / least popular lines and routes, including number of entries and exists, schedules' adherence, types of buses and subway cars in use, etc. This is for all bus, subway, trackless trolley, ferry, and THE RIDE routes.
Top 25 subway stations
Top 25 bus routes
To download your own copy, click on this link. (Warning, .pdf)
Oh, and here's an interesting tidbit of information. The T estimates that upwards of 1/3 of riders use an app to check bus/subway schedules on a regular basis. (Source: TransportationNation.org)