Anna's has 6 locations around boston, all are good.  They make almost any kind of burritto and they make it quick, make sure you know what you're getting when you are next in line.  Thier quesadillas and super burrittos are my favorites (steak, carnitas, al pastor, chicken, etc..) but hey also have tacos and mexican plates.  I can barely finish the super burritto, hearty portions. Their burritos are entirely customizable and easy to try to something new everytime.