Do you want to go into Boston itself? If so there's a lot of options.
Sunset in Allston probably has the largest selection with 100+ taps. Lord Hobo in Cambridge has about 35, Bukowski's has about 20, Cambridge Brewing Co. is one of the oldest brewpubs in the country and has a good selection at all times (very close to Hobo), Meadhall is near Cambridge Brewing and has a very large tap list (50ish I think), Deep Ellum is near Sunset and has a good selection, and Publick House in Brighton/Brookline has a really great belgian selection.
As far as outside of Boston, I used to live in Framingham and never found anything to be very good. BBC is okay and John Harvard's is okay too. I'm not super familiar with the northern part of the state.