Brookline and Arlington have gifted programs, although Brookline has changed its name to a "Enrichment and Challenge Support Program".
Of the school districts you mentioned, ignoring issues such as commute and house prices, I would recommend consideration of Lexington. It does not have a gifted program but does have many gifted children and participates (and does well) in many academic competitions, such as math leagues.
Lexington is quite liberal and has for example a diversity curriculum in elementary school
http://www.wickedlocal.com/lexington/news/lifestyle/columnists/x125181726#axzz1Mjd0DBcm . You can decide if it is a good fit.
