When a sash is beyond salvage (which is rare), I have a new one custom-made from Spanish cedar. All-heart redwood or cedar are also equally good species for this application if Spanish cedar is unavailable. The wood needs to be stable and rot resistant, which rules out most species commonly available today. The first-growth red cypress that was universally used until the mid-20th century in New Orleans, where I work, is hard to beat, compared even with today's finest woods.
For the retrofit project I'm working on now, I ordered new custom-made sashes by G&W Millworks Inc. (504 833-5146). The Spanish cedar was straight-grained and dense, providing a stable platform on which to work. It tools easily, holds paint well, and is as rot resistant as any wood short of genuine mahogany, which is more expensive. Spanish-cedar dust is pungent and can cause reactions for some people, so I always wear a mask when working with it to prevent headaches. On the whole, though, it is a lovely wood to work with.
If the new sash tips the scales more slightly than the replaced unit, this common problem can be addressed by adding small slip-on cylinders, such as those from Blaine Window Hardware Inc. to the existing weights.