The two busiest lines in Boston are the 66 (14,676 riders) and the 39 (14,405 riders)
source. Overall Boston just isn't a city that buses work very well in because of the road network lacking large arterial streets in most of the city because of that the buses are mostly used to feed the subway system. One clear example of this is that combining all the buses that serve Somerville in the area where the Green Line Extension (GLX) will take over providing service they only carry 16,900 riders while the GLX is expected to carry 52,000 riders per day in the same service area.