What is the best replacement window?

The best replacement window, by brand, is a matter of opinion. Some, like Andersen, Marvin and Atrium, have a national reputation, but opinions among homeowners and contractors vary widely. Contrary to what one might expect, price is not necessarily a good measure of a replacement window's quality for all buyers.  

In general, however, a good replacement window will be an improvement on the old window it is replacing, and the best replacement window will be as energy efficient as possible. Windows are notorious weak spots in a home's seal. Heat is lost in winter or gained in summer through the glass. Good replacement windows should have at least double glazing (preferably triple), low-e coating and argon gas between the panes.  

There are ratings listed by the National Fenestration Ratings Council (NFRC) on every window manufactured in the USA. The best replacement windows will have the best NFRC ratings. Of course, the Energy Star label is always a good sign as well. Here is more information on how to decode energy performance ratings.