The Okna 500 is a nice slimline window with great ratings.
You should also look at the Okna/HiMark 800. It's a pretty window that has some wood window attributes like real metal handles.
Here is a pic below of an 800 sample so you can see what it looks like. The sample is in a natural finish but is also available in white and a couple more wood-grain colors. This sample has brushed nickel hardware but it is available in antique brass, adobe grey and white too. Wider windows will have 2 handles top and bottom.