As a lifelong MA resident, I can tell you that without question, the must-see 4th of July event in New England is the Boston Pops Fireworks Spectacular on the Esplanande along the banks of the Charles River in Boston. It is an amazing event and one of the biggest (maybe the biggest) July 4th celebration in the country. However, it can be a zoo. There are literally hundreds of thousands of people that go to, and they are usually there all day. That being said, it is a great time and an absolutely amazing show. And if you want to make it even better, rent a boat (or find a friend with one) and watch it from the river. Looking out and seeing all the people on the banks of the river is one of the most surreal experiences you will ever have.
Here is the website:
http://www.july4th.org/
There are also some other great events depending on what part of New England you will be in.
Boston Harborfest - http://www.bostonharborfest.com/ - It's more of an all-week thing than a one day event, but it's a great time if you are looking for stuff to do before and after July 4th.
Bristol, RI July 4th Celebration - http://www.july4thbristolri.com/ - It's the oldest July 4th celebration in the country. It's not as big or as flashy as the Boston events, but it is a really fun time.
In addition, there are local fireworks displays, clambakes and events all over New England, especially at the beaches (Newport, RI is also a great place to celebrate the 4th). But if you are looking for the truly can't-miss, once in a lifetime event, the Boston Pops Esplanade celebration would undoubtedly be way to go.