It's hard to site any single carpet market as the best or biggest in Marakesh. The city is completely filled from end to end with markets packed full of gorgeous carpets woven in local shops and factories. A top quality carpet from any of these markets should cost no more than 300 to 500 dh per square meter, though you could spend just 200 to 300 dh if you're willing to compromise a little. Prices are generally fair, but Moroccan vendors are famous for their love of the haggle, so you might want to brush up on your deal making skills if you want to walk away with a good carpet without overpaying.
Various styles are available, many being imitations of other country's styles and techniques, the best, however, are those made with indigenous patterns, style and technique, though. These should be easy to spot, but if not, the vendor should be willing to advise you.