What to specifically look for in windows based off of geographical areas and climate zones:
Northern - Look for a better U-value and insulating value of the window. Triple pane may be a worthwhile consideration to draw the U-value down.
North-Central - Depending on your situation, a good quality double pane glass with a respectable U-value is probably your best investment. Triple pane tends to be an overkill for this geographical climate and adds tint, darkening the view. A solar heat gain coefficient rating of between a .25 and .35 would be most effective in this climate.
South-Central - In this climate, nothing more than double pane would be necessary. A lower solar heat gain rating will begin to be important here to allow less heat in the windows and maintain cold air in the house.
Southern - The solar heat gain rating is going to be essential for this region.
For all geographic locations - Air tightness and performance grade are very important. You may want to use different glass packs for certain rooms or sides of your house.