This article is pretty accurate as it is more of a region rather than a city. i don't think any 1 city would capture it. I would consider Worcester to be the central MA sub forum.
Springfield is south, on the CT border and though it is the largest city, would not capture the rural feel of Western MA. Greenfield is north and the same would apply.
Would the labels be better served as:
Cape and Islands
I am not certain how it would be best for search engines though.
Western Massachusetts -