Pay attention to the locking mechanisms, the frame strength and of course the E rating (so you can collect your free money)
. Each "good" company offers crap models and offers good quality windows. Never buy something that the dealer doesn't have a sample window in stock of. If you aren't putting them in yourself then get in writing EXACTLY what they will do to prep the window (i.e. removing old and re-calking, foam crack filling, pulling the trim and insulating properly). The best of window isn't much better than the worst if you don't have the surrounding sealed properly (and if you live in the neighborhood I think you do then your old enough it doesn't meet the current energy code).
Once you think you've narrowed your choices down (and don't limit yourself to the big names like Anderson or Pella) then find out how long the dealer has been selling the window then go on the website for the window company and see how far back you can get parts for their out of date stuff.