You can either insulate the existing doors yourself or buy insulated doors.
Often a door to an unheated garage or basement was never intended to keep the heat in. In addition to weather-stripping these doors, add insulation to the outside. Foam panels are the easiest way to do this, but they must be fire protected. I recommend tacking a frame the thickness of the foam onto the door, and covering it with a piece of 6mm (1/4 in.) masonite-type pre-finish paneling. Don't forget to allow clearance on the hinge side if it open out, and on the catch side if it opens in.
Doors with glass panels in them are beautiful, but lose as much heat as windows. They can be thermally upgraded by removing the glass or adding more insulation. Doors with wooden panels can take insulation sandwiched between the two panels -- but that usually gets too thick to look good.
Solid wood doors are better than panel-type doors but not as effective as thermal doors.
Thermal doors are doors with insulation inside them and the decoration on top. These doors can make the best energy sense in Canada. However, you can rarely justify scrapping a good wood door with good weather-stripping to replace it with a thermal door. If you are buying a new door, get a thermal one -- especially if it's for the north, east, or west wall.
Patio doors are really oversize windows. I have seen one very good European-type sliding door that drops onto its weather-stripping when you close the latch (expensive of course). But other than that, sliding doors, like windows are difficult to seal properly and maintain the seal. On the south side of the house they can be net heat gainers -- perhaps even overheaters. (search keyword "Solar Energy" for the title "WHAT IS SOLAR OVERHEATING?")
If you insist on keeping these beautiful holes-in-the-wall elsewhere, study window heat losses described in the overview "Insulation & House As A System found in the Nuts & Bolts section of this site.