I have been looking at windows for 2 months now in Toronto, Canada for a new home I am building this year and get more frustrated by the day. I can not seem to get a consistent story from the window manufacturers and dealers, Energy Star requirements in Canada are clear, but when I look at the performance data published by the window company I can not often find ER or R value ratings. Unfortunately the NRC's Office of Energy Efficiency web site that I think provdes comparative data for window manufacturers is out of service. I have looked at the NFRC ratings but they do not show R value or ER ratings.

Currently I am looking at Pella, Andersen, Marvin and Thermotch windows.

Could anyone help me with what 3-5 performance factors really are critical when purchasing a energy efficient window and what range I should be looking at for a good  and above average window. Keep in mind that cost does matter. My home will be in Zone B near Ottawa, Ontario. I have great southern exposure which I will be taking advantage of by having lots of windows on the south facing side of the house. The home will be a two story home with a walkout finished basement. Lot's of windows to take advantage of the water view and passive solar heat.   

Or even better if someone has already done a similar analysis, what window did you go with?