Marvin Windows is currently running magazine ads asking well-known designers this question and showing pictures of the pretty windows they came up with:

I really wish they'd ask a few American green-building gurus this question, because I'm sure they'd get an earful:

• Whole-window U-values lower than 0.17, and glass U-values around 0.10 (the window in the Marvin ad has a U-value of 0.30, which is totally lame)
• Solar heat gain coefficient (SHGC) greater than 0.50
• Visible transmittance (VT) greater than 0.50
• Tilt-turn operation with inward swing
• Geeky details I won't bother explaining, such as 4/16/4/16/4mm triple-glazing and a plastic warm edge spacer of Psi=0,038

In Europe, dozens of manufacturers make windows like that, but in North America hardly anyone does. The best North American window vendors are making a reasonable attempt, but they all fail on at least one of Ted's and my requirements:

• Serious Windows' highest-performing windows are casement, not in-swing, which means they open outwards with a little crank at the base. This is mechanically inefficient, and it also limits the size of windows they can sell. We have some gloriously big south-facing windows in our dining area, and we don't want to make them smaller. Also, their best SHGC is 0.42, which is not as good as the standard Passivhaus windows from Europe.
• Canadian companies Accurate-Dorwin and Thermotech Fiberglass didn't sell in-swing windows last time I checked. My main complaint about out-swing windows is that the screen is inside the window, which I simply don't like. And yes, I know about retractable window screens, but I'm still not jazzed about opening a heavy triple-paned window with a crank.
• Inline Fiberglass makes a tilt-turn frame, but they still aren't big enough for our front windows. They offered to sell us a modified doorframe full of glass, but c'mon! Also, their best windows use Serious glass, which has slightly lower SHGC and VT than the European glass.

I could list a few more North American vendors, but they are mostly using Inline frames and Serious glass, whose shortcomings I listed above. And you'll notice I didn't mention the big-name American manufacturers -- as far as I know, Marvin, Pella, and Andersen aren't even trying to build Passivhaus-worthy windows.

Ted and I are therefore likely to order windows from Europe. If money were no object we'd want something like Optiwin's Passivhaus-certified three-wood window, but sadly we are on a budget, and ever-plummeting dollar doesn't help. But the high-end American windows I listed above aren't cheap either, and there's enough variety and competition in Europe, particularly from the former Eastern bloc, that we can get something good for an acceptable price.

We haven't made our final decision yet, but we're likely either to get German windows manufactured in Slovenia (or possibly Slovakia?) or Polish-made windows. We rejected some Lithuanian windows, not because we have anything against Baltic states but simply because the importer is based in DC and we want to work with someone more local. If we're handing someone a five-figure check just to place the order, we want to be able to drive over and hassle them from time to time.

We're still waiting on a couple more estimates, but once we make our decision I'll post all sorts of titillating window specs and diagrams for your perusal.

[Added on 2011-10-03: It's possible I've been a little too hard on North American glass manufacturers, since there's apparently some difference in how they test glass performance in Europe, but I stand by what I said about tilt-turn vs. casement operation.]