I'm the window installer, and, like I said before various manufacturers make windows with higher R values. Just depends if you want to pay for all the bells and whistles. A good Energy Star rated window(around R-3.5) with a good installation is the most practical. Add a bell, add a whistle or more.
On the Serious website it shows they make various windows starting with R-3.3 rating and going up and up to just one window with R-11 rating. (wonder how much that cost?)
That's their Top of the Line fixed picture window. A non-operating window(does not open)
Sue must have a gigantic house for her to have ALL picture windows---with R-11 ratings, yet.
That means no fresh air. Those windows don't open.
And Sue must have three...

I install windows for a living. In truth, many manufacturers have windows with R-11 ratings. Yet most homeowners don't want to buy the "Cadillac" and prefer the "Chevy" with the R-4 rating.
The typical single pane window has a rating of R-1 and that's all.
A dual pane insulated unit with "Low E" coating inside the glass boosts that up to a R-3.5 rating.
Most mannys also include a gas inside such as argon--which boosts the rating up to around R-4.
To get the higher "R" ratings, have both sides(insides, of course) of insulated unit glass treated with the Low E coating.
The most important thing is---good installation. The new window is always measured smaller than the original in the house-for ease of installation.
Low...