Boston is not a great deli town. It is, in fact, a rather lousy deli town, especially if you're used to great deli food like they have in New York (or Detroit or LA or even Chicago).
Michael's Deli is located at Harvard and Beacon. They import the corned beef from NYC. It tastes like real corned beef, with the proper mouth feel that is slightly salty, slightly fat and creamy. They make their own turkey, which is excellent.
Sandwiches are 8 to 9 ounces of meat. They use good breads - especially when compared to the low grade breads used in many Boston deli's.
The pickles are excellent. The knishes, which are huge, cost only $2.25 and make a meal. I prefer the potato knishes, which are rich but not leaden.
We have bought food in bulk for parties. All I can say is buy more corned beef because people go nuts for it. If you need a real deli fix, Michael's is the place.