Before I posted the above message, I had gone to a high-end dealer which carries Margin, Pella, and Loewen, as well as other brands; they recommended Loewen for my application.

After I posted the above message, I looked at the Energy Start Most Efficient web page and Loewen was missing. Any idea why?

Marvin Ultimate is a great wood window. The others are average.
The most energy efficient will no doubt be high end vinyl. Before you jump to conclusions regarding vinyl, the high end brands look very nice plus they are much more energy efficient// air tights, and have more structural integrity than any wood window.
The higher end vinyl windows are : Okna ( EnviroStar or 500 series )
, Soft Lite ( elements or LS) , Sunrise ( vanguard or restorations), Gorell ( 5300 ) , Affinity ( Ultra) , kensington ( Quantum 2 ).

I have sold them both, Loewen makes a very good product. That being said my favorite wood window is the Marvin Ultimate which in my opinion is the best commercially available wood window in the market today. I would rate Kolbe pretty close with Loewen being right there as well. I am not as well versed in vinyl so I will leave that for others (mmarse, homesealed, windowsonwashington, et al).

I'd also check into Kolbe EP casements. Price point might be a bit better than Loewen but same quality. Loewen, Marvin and Kolbe are all good choices. Just have to see what fits in your budget and what one you think looks best. Pella in my opinion probably little under the rest in quality.

+1 to Marvin being the best largely available wood windows out there.

Great window and you are going to be hard pressed to find a better window in wood out there.

As long as its the marvin ultimate, not the integrity.

Yes mmarse I was speaking of wood or wood/alum clad not Ultrex.

I'd agree with the others on wood window recommendations. Marvin and Kolbe are my favs, and the Loewen stuff is nice from what I've seen. That said, the number one priority that you listed was "the most energy efficient windows that I can afford". None of these choices are going to fit that bill. Premium vinyl, composites, and some fiberglass options will have much better performance than pretty much any wood window. Look closely at u-value and air infiltration ratings.
Wood windows are a great option and the right fit for many people, but you are generally sacrificing performance for appearance. It depends on what is more important to you.

How does the U-value and air infiltration numbers compare between the Marvin, Kolbe and Loewen?
Condensation resistance can be very important on wood windows as well since they are more prone to water damage.