depends on how much you want to spend.

super cheap, but good stuff? go out to framingham, simple living cycles. (57 waverly street, framingham, ma) guy is slightly nutty, but funny.

want to spend lots of money for a "good cause"? go to bikes not bombs. (18 bartlett square, boston, ma)

lots of money with brains behind the workers? harris cyclery. (1353 washington street west newton, ma)

want hipster, super polished product display? go to superb. (824 beacon street, boston, ma)

want to see a shop with nothing in it but bikes that need to be repaired? hub bicycle, corner of (columbia and cambridge street, cambridge, ma)

pretend "co-op"? broadway bicycle (351 broadway, cambridge, ma)

getting lazy, so here are the other popular places, depending on your taste: revolution bicycle, (good for "bomb proof wheels") arc-en-ciel (bike "studio"), belmont wheelworks (big shop), international bicycle (big shop), cambridge bicycle (mildly big shop), ferris wheels (never been), and i know i'm missing other townie/weird shops most people don't go to anyway. you're welcome, and don't open a bike store in boston.

ps: if anyone on the forum works at any of these places, don't hate me. it's the internet.