There are a few places in town that do a great job of educating you before you purchase, International Gemological Labs Calgary is a good start (in Bridgeland), so is Alberta Diamond Exchange (in Aspen Landing).
For pricing, the Rapaport report is the international pricing guide, sets the price in USD per carat based on color and clarity (there are different reports for different shapes/cuts). This will help you to know what kind of deal you are getting (if any).
IGL
Alberta Diamond Exchange
diamonds.net Rapaport Report
If you want the most brilliant sparkling diamond, the best will be an IGA certified (not EGL their standards are looser) "Ideal cut", and minimum Excellent Symmetry and Excellent Polish (note all round cut diamonds feature hearts and arrows, the better the cut the more symmetrical the hearts an arrows will be), and as close to colorless (D color) as your budget can afford in the size you are looking at. Don't bother looking at anything with clarity lower than a VS1, and make sure there are no florescent's (need to look at the diamond under a black light to test this).
Note, if you are buying a round brilliant diamond for someone whom is superstitious, make sure the Culet is cut (the point on the bottom) as it creates a better number of cuts over all (58 cuts rather than the normal 57).
I also strongly recommend having the girdle of the diamond laser etched with its serial number to ensure you have and continue to have the diamond you purchased. Do not let it out of your sight until it has this serial number. Also, make sure to have it appraised after it is set and added to your home insurance.