I like Yeffer's suggestions, however, Pat's Peak can get really crowded. It is very popular with school programs and a lot of kids get dropped off.
I have skied at all of the ski areas listed and Crotched in my opinion is the the one I would choose. It is more cost effective. If cost is not a real issue - then Sunapee has a great deal that lets you ski there, Okemo and Stratton on a single season pass. Obvoiusly, the other two are further away, but are great getways!
However, there are two others that are around the two hour mark from Boston and are considerably better mountains - Ragged and Gunstock.
Ragged is actually a decent ski area and have great instructors. One of my favs!
Gunstock is also pretty decent as well - the only issue - it is a hike to the chairlifts from the base lodge and you have to plan how you want to get over to another chair - wrong trail then you are stuck back at the same lift.
All in all, I am not a big fan of season passes since we bring our kids to over a dozen ski areas per season and we ski about 20-25 times per season. Not sure what your plans are, most season passes break even at around 10 - 15 trips - Pats, Crotched, Wachussett, and Nashoba could get old real fast. No offense to any of the ski areas (I ski them every season), but variety is key - I wouldn't ski a bigger mountain that many times.