Ooh, so many opinions.
We live in Dorchester and love it. It's right on the red line, there are some good public schools, our neighborhood has lots of kids... But Dorchester is geographically huge and very diverse. Not all the neighborhoods are nice, and really nice can be right next to really dodgy. Like I said, we love it, but some people worry.
Brookline has famously good public schools, and parts of it are on the Green C line. Other parts are quite close to BU.
Newton is another one with awesomely good schools.
Every time I go to Somerville (the north end of the red line), I run into friendly parents with kids. There are great parks and playgrounds there. Don't know that the public schools are all that though.