Every climate is different, and the direction your window faces comes into play as well, so consult your salesperson for the best choice when it comes to efficiency. In general, you'll want a window that has two or three panes of glass with space in between. The pillow of air between the panes acts as an insulator. Gases like argon are an even better insulator. Look for a film on top of the glass that reflects the hot summer sun on the outside and keeps heat in during the winter.
This corner window is so cool though I don't imagine we'll be able to fit it in on our budget. But, at the very least, I would like a window over the kitchen sink - makes the task of washing pots & pans more enjoyable.
Wow, so much natural light! This seamless corner window installation creates an instant focal point in this stunning space. Connecting the glass with a clear caulk, rather than a frame, made all the difference.