Author: Window4U (IL) (---.client.insightBB.com)
There are other important factors to consider than just u-value.
Any window near a U-value of .20 is basically the same in that category and there are many windows in that range.
Other factors such as DP ratings, air infiltration rates, the windows features and the fit & finish all contribute to the real world longevity and performance of the window.
I doubt that the three you mentioned are the 3 top u-value performers, though Schuco is close. The difference between a window that is a .19 compared to a .20 or a .21 is negligable and can be easily offset by any of the other factors.