I am often asked "what are the best replacement windows?". There is no simple answer. For a through discussion check out the National Resources Canada publication Energy-Efficient Residential Windows, Doors and Skylights, which can be found with many other energy efficiency publications at http://energy-publications.nrcan.gc.ca/index_e.cfm.
What you must keep in mind is that every type of window has cheap and quality products on the market, so don't believe that just choosing the material that the frame is made of will end your research. Aluminium and fibreglass frames are probably the most durable, which explains why most rental units have aluminium windows. Aluminium will loose the most heat through the frames, despite thermal breaks that are in almost all Canadian aluminium windows. Fibreglass frames are probably the best in the market, but also the most expensive and hard to find. Click here for an update on fiberglass windows. Wood requires maintenance, but you can get vinyl or aluminium claddings on the outside of the wood window to eliminate the outdoor maintenance. You will still have occasional scraping and refinishing to do on the inside, although wood is probably one of the best for having warm frames. Vinyl is the most common because it is good value for the dollar, but here especially you will see a wide range of quality.
One of the things that I look for when searching for quality is the hardware itself. If it is sturdy and well moving, that is a sign that the rest of the window is probably well built. If the hardware is loose and sloppy or even appears cheap, the window is probably of the same quality.