My bet would be East Brookfield, West Brookfield, and Palmer. Palmer is obvious - the single largest population between WOR and SPG, and close to the Pike. The Brookfields and Warrens are tricky because you have five distinct villages (four separate municipalities) on the line, plus three off the line. West Brookfield plus East Brookfield puts all eight villages within a 10-minute drive of a station, and both sites have ample room for parking. But how many stops in Warren and the Brookfield, and where, probably depends more on local politics than what's theoretically ideal.
By point of comparison, by 1959
(just before all local stops were discontinued), most Springfield and Albany trains stopped only at Palmer between Worcester and Springfield. One daily round trip (timed for day trips to Boston) also stopped at Warren, West Brookfield, East Brookfield, and Charlton.
The main intercity stops at that time were South Station, Huntington Avenue/Trinity Place (Back Bay), Newtonville, Framingham, Worcester, Palmer, Springfield, Pittsfield, Chatham, Albany. I would probably swap Newtonville for Wellesley Square, and add Westfield with its state university.