Best bang for your AR-15 buck is a used Norinco for ~$500-$600 depending on shape/parts. Works fine, accurate, cheap, ugly finish/furniture, which doesn't affect servicability.
A question this vague will invariably end up with a parade of 'what I own is best' answers.
For AR-15s, in no particular order, Armalite, Bushmaster, Stag Arms, Rock River Arms, DPMS (newer), and Dlask are all good and (fairly) reasonably priced on the Canadian market. I don't include Colt, good rifles, but if bang for the buck is the question, Colt isn't the answer.
Find out what you want (read here and ar15.com). Compare apples to apples for pricing on the features you want. Best thing would be find a local dealer you like to work with. There are no 'bad' ARs being distributed in Canada today.
There are also about 5 active threads comparing various black rifles.
Hope that helps.