I’m a bit biased, but Slush King in East Boston is pretty damn perfect. It’s just an old school takeout window/walk-up. Nothing over the top. Exactly what you need at a reasonable price.
Otherwise, Cabot’s in Newton is the gold standard. It’s what movie set designers imagine when a director says “ice cream parlor”. (I haven’t been since it got new owners though).
Toscanini’s for ‘fancy’ ice cream. Their B3 flavor is a favorite.